ga.io is giving improper date formatting for yearMonth dimension
## INSTALLED VERSIONS

commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None

pandas: 0.13.1
Cython: 0.20.1
numpy: 1.8.0
scipy: 0.13.3
statsmodels: 0.5.0
IPython: 1.1.0
sphinx: 1.2.1
patsy: 0.2.1
scikits.timeseries: None
dateutil: 1.5
pytz: 2013b
bottleneck: None
tables: 3.1.0
numexpr: 2.3.1
matplotlib: 1.3.1
openpyxl: 1.8.2
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: 0.5.2
sqlalchemy: 0.9.2
lxml: 3.3.1
bs4: 4.3.1
html5lib: None
bq: None
apiclient: None

I have a list of tuples which contain date ranges, like so: 

```
[('2012-01-01', '2012-01-25'),
 ('2012-01-26', '2012-02-20'),
 ('2012-02-21', '2012-03-16'),
 ('2012-03-17', '2012-04-11')...]

def pull_unsampled(date_list):

    df123 = []

    for start, end in date_list:

        df123.append(ga.read_ga(metrics, 
                    dimensions = dimensions, 
                    start_date = start,
                    end_date = end, 
                    account_id = account_id, profile_id = profile_id))

        print start, end

    return df123

print pull_unsampled(unsampled_date_ranges)
```

The above function appends each GA pull for each date range to a list.  For some reason the yearMonth column is formatted improperly until 2013 date ranges are reached.  For example, the first date range of ('2012-01-01', '2012-01-25') returns this:

```
[                                           visits  transactionsPerVisit  \
deviceCategory operatingSystem yearMonth                                  
desktop        (not set)       2020-12-01     137              0.000000   
               Chrome OS       2020-12-01      20              0.000000   
               Google TV       2020-12-01       5              0.000000   
               Linux           2020-12-01     363              0.275482   
               Macintosh       2020-12-01  111467              0.909686   
               Nintendo Wii    2020-12-01       2              0.000000   
               Playstation 3   2020-12-01       1              0.000000   
               SunOS           2020-12-01       3              0.000000   
               Windows         2020-12-01   98147              0.996464   
mobile         Android         2020-12-01    2976              0.302419   
               BlackBerry      2020-12-01    1051              0.000000   
               Nokia           2020-12-01       2              0.000000   
               Samsung         2020-12-01       1              0.000000   
               SymbianOS       2020-12-01       4              0.000000   
               Windows Phone   2020-12-01      46              0.000000   
               iPad            2020-12-01       2              0.000000   
               iPhone          2020-12-01   24754              0.379737   
               iPod            2020-12-01     331              0.000000   
tablet         Android         2020-12-01     189              0.529101   
               iPad            2020-12-01   24973              0.828895 
```

And the last date range , ('2014-03-06', '2014-03-31'), returns this:

```
deviceCategory operatingSystem yearMonth                                    
desktop        (not set)       201403          1.127708           1.013683  
               Chrome OS       201403        159.150562           3.811236  
               Google TV       201403          0.000000           1.000000  
               Linux           201403        431.220165           8.705575  
               Macintosh       201403        219.625070           5.193008  
               Nintendo Wii    201403          0.000000           1.000000  
               Playstation 3   201403         65.000000           3.000000  
               Windows         201403        186.650353           4.077768  
               Xbox            201403          0.000000           1.000000  
mobile         (not set)       201403         51.151515           2.212121  
               Android         201403        143.776396           3.195142  
               BlackBerry      201403        132.644037           3.049541  
               Nokia           201403          0.400000           1.000000  
               Series40        201403          5.071429           1.214286  
               SymbianOS       201403          0.000000           1.000000  
               Windows Phone   201403        172.685446           3.305164  
               iOS             201403        126.005593           2.954413  
tablet         Android         201403        183.609062           3.746048  
               BlackBerry      201403        224.421053           3.789474  
               iOS             201403        217.915084           4.462462  
```

I did some testing and it does not seem to be an issue with the GA response, and likely and issue with the ga.io request.
