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.

any idea why this doesn't show the same warning? (this is not in cython)

```
(py3.5)bash-3.2$ python -Wall
Python 3.5.0 |Continuum Analytics, Inc.| (default, Oct 20 2015, 14:39:26) 
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from re import compile as re_compile
>>> re_compile('\s+')
re.compile('\\s+')
```
 @jreback This doesn't affect `re.compile()` since to it `\s` is a recognized escape sequence in the context of a regular expression.  In a replacement pattern to `re.sub`, however, it is not a meaningful escape sequence (in the regular expression language), so it needs to be passed literally `\\s` in order for it to replace with the literal `\s`.
 thanks!
