Plot degree labels in TeX mode

Use Unicode character or TeX math mode depending on whether `tex.usetex`
is enabled. Avoids exceptions like the following:

    UnicodeEncodeError: 'ascii' codec can't encode character u'\xb0' in
    position 290: ordinal not in range(128)
added method to set coord_type and image test
Fix bugs with CoordinateTransform
Merge pull request #70 from anizami/set-coordinate-type

added method to set coord_type
Merge pull request #71 from astrofrog/coord-fixes

Minor fixes to CoordinateTransform
Make get_coordinate_system extensible
Fix issues when spacing is not a multiple of base spacing
Added unit tests and fixed failures
Fix failure with old versions of Numpy
Added unit test for get_coordinate_frame
Merge pull request #73 from astrofrog/minor-fixes-formatter-rounding

Fix issues when spacing is not a multiple of base spacing
Simplify reset_frame_identifiers
Merge pull request #72 from astrofrog/extend-get-coord-system

Make get_coordinate_system extensible
Merge pull request #58 from lpsinger/tex_labels

Plot degree labels in TeX mode
Added unit test for #58
Added CHANGES.md file
Remove duplicate LICENSE, updated MANIFEST
Minor fixes to docs
Added developers/contributors