Getting started: Update for stb-tester 0.14

stb-tester 0.14 no longer ships GStreamer elements (all the image
processing code is done using the OpenCV python bindings).

Note that I removed the "GStreamer caps" section because I no longer
have a good example element to use to explain caps.

This commit also contains miscellaneous updates to reflect the latest
Fedora and OS X package namings, etc.
Getting started css: Fix vertical spacing of footnotes 1 & 5