change default from np.median to np.ma.median
Remove parallel testing from Travis, since it is prone to many false positive failures.
Update rotation_matrix to reduce rounding error and correct sign convention on for arbitrary axis
Add explicit rotation_matrix tests
Make angle_axis more consistent and numerically stable (and test)
Add changelog entry
array -> asarray as suggested by @mdboom
Updated tests following @neothemachine's suggestions
Added option to use representation names in represent_as
Added test for represent_as
Replace class attribute frame_attr_names with descriptors
Update docs
Add unit tests of FrameAttribute descriptor
Put FrameAttribute into public coords API so it shows up in docs
Restore proper inheritance of frame attributes via __mro__
Change frame_attr_names() to get_frame_attr_names()
Merge pull request #2615 from astrofrog/remove-parallel-travis

Remove parallel testing from Travis
Add coveralls --rcfile option
First pass at "model sets" implementation.  This gets all the existing
tests working.  Some tests had to be updated to *explicitly* set
model_set_axis=0.  This is because they used model types that previously
set their param_dim implicitly from the shape of the first
parameter--the type of ambiguous implicit action we're trying to get
away from.

As a result I believe there were some tests that intended to test single
models (i.e. param_dim=1) with array-valued parameters, but didn't
actually do this.

As such, array-valued parameters are not yet working quite properly, but
that will come in the next changeset.

Another annoyance with this so far is that in order to use
model_set_axis to specify the set size it is *necessary* to provide a
value for at least one parameter.  This seems arbitrary and awkward for
models that have default values for all their parameters.  It should be
possible to initialize those without providing any parameter values.

For that reason it might be worth introducing an n_models argument with
the same semantics and param_dim (some people in previous discussions
were okay with the name 'n_models').  I'd also consider model_set_size.
model_set_size could work in tandem with model_set_axis and one, the
other, or both could be specified (if both they would obviously have to
agree).
Fix the param_sets property to work somewhat consistently within the new system.  I'm not sure how much longer I want to keep this property around in its present form at all, but for now I tried to manimize the changes necessary to any other code or tests.  One thing that is slightly different now is that *previously* the last axis of the param_sets array was always the same size as the number of param sets (that is, the last axis was the param set axis).  Now that is no longer guaranteed--instead it corresponds to whatever axis was specified by model_set_axis.  We could roll that axis to the end, but for now I prefer preserving the contiguity of each parameter array).