Merge pull request #1072 from mdboom/unit/more-physical-types

Add more units and physical types
Expose parallel testing support at the command line
Remove the global state from the table tests so that they can be run in parallel
Merge the test_timer tests into one function since they must be run sequentially to succeed.
Fix a sometime race condition in the tests whereby `FOOBAR` would be a registered unit and therefore would not work as expected when setting to a `wcsprm.cunit` list.
Try to solve the unit definition problem
Add a custom version of catch_warnings that clears all warning state first so that we can reproducibly get all warnings everytime.
Turn on parallel testing on Travis
Install pytest-xdist on Travis
Disable parallel testing for Python 3.3
Add a section to the testing guide describing the parallel option
Make documentation a little more specific