Import ANCHOR_SORT in delphin/mrs/ep.py. It was forgotten earlier.
Added basic unit tests.

I added some tests for delphin.codecs.simplemrs and delphin.mrs, and also
one for delphin.mrs.semi, which is not complete (neither the tests nor the
module). I then made minimal changes to the code to get the tests to pass.
The tests for all modules are incomplete.
Add some more unit tests for delphin.codecs.simplemrs.
Add some more unit tests and fixes to handle them.

The unit tests are still incomplete, but this adds some more.
Make Mrs and Dmrs into functions, and fix unit tests.

There are many changes here. The initial change was to make Mrs() and
Dmrs() functions instead of subclasses of Xmrs, which necessitated some
changes, but I took the opportunity (perhaps to the detriment of an
atomic commit) to make Xmrs conform more to the API published on the
GitHub wiki (https://github.com/goodmami/pydelphin/wiki/delphin.mrs).