BUG: Timestamp.replace() behaves naively at DST boundaries (plus bonus segfault!)
``` python

In [1]: import pandas as pd

In [2]: t = pd.Timestamp('2013-11-3', tz='America/Chicago')

In [3]: t
Out[3]: Timestamp('2013-11-03 00:00:00-0500', tz='America/Chicago')

In [4]: t.replace(hour=3)
Out[4]: Timestamp('2013-11-03 03:00:00-0500', tz='America/Chicago')
# This time doesn't exist.

In [5]: pd.Timestamp('2013-11-3 03:00:00', tz='America/Chicago')
Out[5]: Timestamp('2013-11-03 03:00:00-0600', tz='America/Chicago')
```

In trying to replicate the definition of `Timestamp.replace` in [tslib.pyx](https://github.com/pydata/pandas/blob/320f9d4c44d60cc4ae5e08ab4a26dc49d3fd71c1/pandas/tslib.pyx#L421), I encountered this further issue:

``` python

In [6]: from datetime import datetime

In [7]: datetime.replace(t, hour=3)
Segmentation fault: 11
```

Trying another tack, I found this:

``` python

In [1]: from datetime import datetime

In [2]: import pytz

In [3]: t = pytz.timezone('America/Chicago').localize(datetime(2013, 11, 3))

In [4]: t.strftime('%Y-%m-%d %H:%M:%S %Z%z')
Out[4]: '2013-11-03 00:00:00 CDT-0500'

In [5]: t.replace(hour=3).strftime('%Y-%m-%d %H:%M:%S %Z%z')
Out[5]: '2013-11-03 03:00:00 CDT-0500'
# This time still never happened
```

In summary: `datetime` is horribly broken, and the current workaround doesn't actually get all the way around.  I know breaking backward compatibility is painful, and throwing the stdlib out the window is even more so, but this situation reminds me of nothing so forcefully as Microsoft Excel's 1900/1904 issue.

Version information:

``` python

In [1]: from pandas.util.print_versions import show_versions

In [2]: show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.5.final.0
python-bits: 64
OS: Darwin
OS-release: 13.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.14.1
nose: 1.3.0
Cython: None
numpy: 1.8.1
scipy: 0.12.1
statsmodels: None
IPython: 1.1.0
sphinx: None
patsy: None
scikits.timeseries: None
dateutil: 2.2
pytz: 2014.4
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.3.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
rpy2: None
sqlalchemy: None
pymysql: None
psycopg2: None
```
