Bug in HCONS dictionary construction.
Add unit tests for MrsVariable and 2 small fixes to var.py:

 * MrsVariable's hash on their string, rather than vid
 * Equality comparisons no longer crash when compared to non-int non-str args
Add unit tests for Hook. Also add equality test for MrsVariable.
Add unit tests for HandleConstraints.
Add unit test for AnchorMixin.
Improve the creation of DMRS links so it's more robust to incomplete MRSs.

Also, SimpleMRS now gives the whole MRS structure a Lnk based on those of
the EPs contained by it.
Add option to print SimpleMRS with color.
Allow MRS codecs to ignore extra keyword arguments.

This prevents MRX and DMRX from crashing with the previously introduced
color arguments for SimpleMrs.

This commit also allows the top-level mrs.py script to accept STDIN as
input for MRSes, instead of just files.
MRS codecs now operate on lists of MRSs.

The load(s) and dump(s) of DMRX, MRX, and SimpleMrs now read and write
lists of MRSs instead of single MRSs. This also means that MRX and DMRX
use the <mrs-list> and <dmrs-list> root nodes, respectively.
Add the extract command to itsdb.py.

The extract command takes a table and a list of fields and outputs
the contents of those on stdout.

This commit also fixes a few small bugs, like for crashing on
constant arguments in SimpleMrs and a missing import in mrs/pred.py.
Merge branch 'master' into MrsPath

Conflicts:
	delphin/mrs/xmrs.py