ENH: Implement core/strings/wrap method
This patch implements the str.wrap function within core/strings.

Example:

```
    >>> s = pd.Series(['line to be wrapped', 'another line to be wrapped'])
    >>> s.str.wrap(12)
    0             line to be\nwrapped
    1    another line\nto be\nwrapped
```

This is a cleaned branch; the original is at #6705.

@jorisvandenbossche ok?

@jeffreystarr looks good, i will rebase/squash you on merge
 yep, looking good! (I think all comments were already adressed)
 merged via https://github.com/pydata/pandas/commit/5b838ab4438435838a09aa0472ef0fd66ec5541a

thanks!
