rebase
add constraints tests
fix linear attribute
remove an erroneous  import statement
Address comments by @iguananaut; add m ore tests
Rebased
remove a commented line (and start a new travis build
Revised the list of models with constraints. Also converted rotations classes to using the new Parameter object (somehow missed this the first time).
Fix to_string with decimal=True
Updated issue numbers in regression tests
Fixed failing tests
Fixed more tests
Merge pull request #1323 from astrofrog/coordinates-fix-decimal-formatting

Fix decimal formatting for Angle objects
Merge pull request #1243 from nden/refactor-constraints

Refactor constraints
Add latex and unicode formats to Angle.to_string()
Add way to only include some of the sexagesimal fields.
Fix some failing tests
Merge pull request #1320 from mdboom/coordinates/angle_to_string_formats

More options for ``Angle.to_string``