add option of "preferred_attr_units"
six.iteritems -> items
ra default unit hours->degrees
fix for preferred_attr_units not being respected
Merge pull request #19 from eteq/coordinates-ape5-prefunits

add option of "preferred_attr_units"
make representation imports at top of module
Remove obstime setters
Added `is_frame_attr_default` and associated machinery
added tests for `is_frame_attr_default`
docstring clarifications
Merge pull request #20 from eteq/coordinates-ape5-defaultfattrs

Method to check if attributes were set explicitly
Initial commit of SkyCoordinate high-level class
Use BaseCoordinateFrame not SphericalCoordinatesBase
Change SkyCoordinate to SkyCoord
Update to use frame_transform_graph
Change COORD to FRAME for globals names
Factor out  determination and allow specification in args
Major refactor of input args, kwargs processing
Restrict transform_to system to be a string and do transform correctly
Change  to  everywhere