Add regression test for the Groningen data model.

Alexander and I found an anomaly whereby our Gronigen-specific data
model definitions would not compile into legal Sawzall due to
unescaped/unsanitized reserved keywords that were intermixed into
the output Sawzall code.  Name "proto", which is in our package path.

The fix converts the keyword list into a set, and sanitizes all output
names.
Merge pull request #7 from arbeitspferde/fix/protocol-buffer-compilation/reserved-escaping

Add regression test for the Groningen data model.