Closes #7758 - astype(unicode) returning unicode.
I didn't have to rely on `infer_dtype` as suggested by @jreback in #7758
as I delegated to numpy to do all the dirty job :
Just calls numpy.unicode on all the values.

Please have a critical look at this pull request before merging :
as I am unfamiliar with both pandas and cython; and may have 
misunderstood the way pandas is working.

Unit tests are attached and seem to work alright on `python 2.7.2` and `python 3.3.0`

closes #7758 

looks pretty good!
pls add note in v0.15.0.txt (with the issue number) - bug fix section is ok
 Done.
 merged via https://github.com/pydata/pandas/commit/a797b28c87d90a439dfa2c12b4a11e62bf0d6db2
 thanks! nice fix
