parallelize_tests

This is a possible solution to #1508 to get the test suite running in parallel.

The main fix was to not use a single gs process to do the conversion but to instantiate it every time.  This makes parallelizing much easier, and it was never much of an optimization anyway.  (Alternately, we could use one `gs` process if not parallelized, but I couldn't figure out how to detect in nose which mode we were in).

There was also a bug where the mathtext tests were being generated with different function names than what they were called in the module's namespace, which broke pickling of those functions through multiprocessing.

On my 4 core i7 machine, the wall clock time was 10:32, now it is 3:05 running all 4 cores.  Pretty nice!

This also adds more tests to the default suite run by Travis.