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`