Timestamp timezone inconsistency
Hi,

Is this a known issue?

Time offset on 22nd of Sept should be -4 hours in New York as below:

```
pandas.Timestamp('2014-09-22 09:30:00-0400', tz='EST5EDT').tz_convert('EST5EDT')
Timestamp('2014-09-22 09:30:00-0400', tz='EST5EDT')
```

but when providing this twice (-0400 and EST4EDT), the result becomes -0500:

```
pandas.Timestamp('2014-09-22 09:30:00-0400', tz='EST5EDT')
Timestamp('2014-09-22 08:30:00-0500', tz='EST5EDT')
```

Thanks,
Mark
## INSTALLED VERSIONS

commit: None
python: 2.7.6.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-37-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8

pandas: 0.14.1
nose: 1.3.0
Cython: 0.19.2
numpy: 1.8.1
scipy: 0.13.2
statsmodels: 0.5.0
IPython: 2.0.0
sphinx: None
patsy: 0.2.1
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.2
bottleneck: 0.8.0
tables: 3.0.0
numexpr: 2.3
matplotlib: 1.3.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999
httplib2: 0.8
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
