Check the python extension in travis

Under the travis environment, the python FF extension is for some reason
installed to /usr/local/lib/python3/dist-packages instead of say
/usr/local/lib/python3.2/dist-packages. The former is not in the default python
site-packages path, so it must be manually added.

testing

more test

Fix typo; expedite python3 path checking

Cleanup the script [ci-skip]
Fix mislabelled, empty string output in U. F. O. output via PyObjectToXML.
Adjust some comments in ufo.c.
Output lib data even when there is no splinechar supplied to to PythonLibToXML.
Merge pull request #1498 from jtanx/pyext-travis

Check the python extension in Travis.
Merge pull request #1618 from fontforge/ufo_blank_lib_1

Fix U. F. O. lib output.
Fix a crash that occurs when one selects no glyph from the picker when kerning by classes.
Merge pull request #1627 from fontforge/metrics_crash_1

Fix a class kerning crash.
Update CONTRIBUTING.md
Small leak and ineffective code GIOGetMimeType()

Interesting result from valgrind revealed that
 - a line of code *was* being executed,
 - but that that line's work was being ignored

Fix changes the line to be an assignment, and then adds the required
intermediate g_free().

This typo didn't alter the final result, as the third (!) call to
routine g_content_type_guess() gave the correct final result.  (I checked)

Still, @khaledhosny might want to check the original code to see if
this typo exists there also.
Merge pull request #1630 from tshinnic/leak_giogetmimetype_wow

Small leak and ineffective code GIOGetMimeType()