Fix import-time DeprecationWarning on Python 3.5
Fixes the following:

```
$ python3.5 -Wall
Python 3.5.0 (default, Sep 18 2015, 09:31:01) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-11)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas.tslib
pandas/__init__.py:7: DeprecationWarning: bad escape \s
  from pandas import hashtable, tslib, lib
```

In Python 3.5 re.sub replacement patterns containing unrecognized character escapes are deprecated.  This raw string format is needed to substitute `\s` literally.
