Fixed bug with array unit conversion with big-endian arrays
16-bit floats don't work with Numpy 1.4 and 1.5 - no need to test.
Add workaround for Numpy 1.4.1 bug: unicode dtypes cannot be used (and we are importing unicode_literals from __future__, which is why we have this issue).
Merge pull request #744 from astrofrog/unit/fix-quantity-big-endian-bug

Issues with big endian Numpy arrays in Quantity
Extract messages from wcslib about rejected (non-standard) keywords and convert them into Python warnings.
Upgrade to wcslib-4.17