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 misspelled word ("Deletetion")
Merge pull request #1317 from vterron/master

Fix misspelled word ("Deletetion")
Fix formatting of Angle objects when precision=0
Use `isinstance(p, numbers.Rational)` rather than `isinstance(p, fractions.Fraction)`.  I have **no idea** why that matters, but it seems to.
Use a unique tuple rather than a hash as the dictionary key -- hash collisions are unfortunately all too common.
Merge pull request #1315 from mdboom/unit/fix-random-failure

Fix random failure in units unit tests
Upgrade to wcslib 4.18
Merge pull request #1305 from mdboom/wcs/upgrade-4.18

Upgrade to wcslib 4.18
Merge pull request #1319 from astrofrog/coordinates-fix-angle-formatting

Fix formatting of Angle objects when precision=0
Fix to_string with decimal=True
Updated issue numbers in regression tests
Fixed failing tests