Add Travis integration

[dvhart]

Add travis integration to install dependencies on Linux and run basic unit tests (which also need to be written). Unit tests should verify at least the following functionality:

$ mkvrjpg.py stereo.jpg
$ mkvrjpg.py -o out.vr.jpg stereo.jpg
$ mkvrjpg.py -o out.vr.jpg -s stereo.jpg
$ mkvrjpg.py left.jpg right.jpg
$ mkvrjpg.py -o out.vr.jpg left.jpg right.jpg
$ mkvrjpg.py -l left.jpg -r right.jpg
$ mkvrjpg.py -l left.jpg -r right.jpg -o out.vr.jpg

It should also verify the following fail gracefully

$ mkvrjpg.py -l left.jpg stereo.jpg
$ mkvrjpg.py left.jpg
$ mkvrjpg.py missing.jpg
$ mkvrjpg.py left.jpg missing.jpg

And more along those lines.