ENH: Implement core/strings/wrap method

This patch implements the str_wrap function within core/strings. The implementation follows 
the behavior of R's stringr library. When a string is 'wrap'ped, the return value will be a 
paragraph with lines of max(word length, width) length.

Example:
```python
>>> str_wrap(Series([u'line to be wrapped', u'another line to be wrapped']), width=12)
Series([u'line to be\nwrapped', u'another\nline to be\nwrapped'])
```