Fixed errors and out-of-date information in README file for OS X
Disable GStreamer support for OS X, to avoid build issues

This means that we won't be able to use frogr to upload videos on OS X,
but everything else should be fine, and the bundle should be smaller.
Removed gtk-mac-integration from jhbuild moduleset for OS X

This is no longer needed since now support for OS X is bundled in GTK+
Update copyright year in Info-frogr.plist.in for OS X
Updated jhbuild moduleset to GTK+ 3.8, add yelp-tools and remove gstreamer support

This should allow building the latest version of frogr without support
for video uploads, which should be ok for now.