Allow Unicode field names for slicing on Py2

Addresses issue #374.  The behavior for field names in slicing arguments is as follows:

1. On Py2, byte or Unicode strings are permitted  (change: added Unicode support)
2. On Py3, only Unicode strings are permitted (unchanged)

(2) is historically the case because of the substantial complexity required to support byte strings when dealing with NumPy compound types on Py3.