Changed angle_units flag to take 'degrees' and not 'deg' in wcs.py.
Update test_wcs.py

Changed deg to degrees.
Made some changes in sunpy.wcs.
1. Changed 'deg' to 'degrees' in wcs.py and test_wcs.py.
2. Added an else clause in wcs.py:convert_angle_units() that raises a ValueError if an invalid unit is passed.
Merge pull request #908 from VaticanCameos/AngleUnitsIssue

Changed angle_units flag to take 'degrees' and not 'deg' in wcs.py.