Using resample() with groupby on this DataFrame causes Segmentation Fault
When trying to resample timestamps into 5 minute time slots grouping on an id column (tried both counting and summing aggregation in 'how' parameter). In a DataFrame with TimeSeries data I get a memory crash, i.e. Segmentation Fault.

I reduced the DataFrame as far as I could in reproducing the crash. Also noted that it will not cause a segfault if I sort the index (don't know if this is needed for resample() function could not find such documentation).

``` python
import datetime
import pandas as pd

all_wins_and_wagers =\
[(1L, datetime.datetime(2013, 10, 1, 16, 20), 1L, 0L),
 (2L, datetime.datetime(2013, 10, 1, 16, 10), 1L, 0L),
 (2L, datetime.datetime(2013, 10, 1, 18, 15), 1L, 0L),
 (2L, datetime.datetime(2013, 10, 1, 16, 10, 31), 1L, 0L)]

df = pd.DataFrame.from_records(all_wins_and_wagers, columns=("ID", "timestamp", "A", "B")).set_index("timestamp")
df_resampled = df.groupby("ID").resample("5min", "sum")
```

Tried on following setups of pandas.
## INSTALLED VERSIONS

commit: None
python: 2.7.8.final.0
python-bits: 64
OS: Linux
OS-release: 3.16-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.14.1
nose: 1.3.4
Cython: None
numpy: 1.9.0
scipy: None
statsmodels: None
IPython: 2.3.0
sphinx: None
patsy: None
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.7
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.0.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
## INSTALLED VERSIONS

commit: None
python: 2.7.8.final.0
python-bits: 64
OS: Linux
OS-release: 3.16-2-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.13.1
Cython: None
numpy: 1.8.0
scipy: None
statsmodels: None
IPython: 2.3.0
sphinx: None
patsy: None
scikits.timeseries: None
dateutil: 1.5
pytz: 2012c
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.0.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
sqlalchemy: None
lxml: None
bs4: None
html5lib: 0.999
bq: None
apiclient: None
