Improve Latests TBB Format to be easily parsable

Currently latests version of TBB is downloadable from https://www.torproject.org/projects/torbrowser/RecommendedTBBVersions and contain a list of latests TBB versions in a format that can lead to difficulties in parsing an deciding "which is the latests alpha release" and "which is the latests stable release" .

That's something that triggered a bug on GeTor implementation in Tor2web described here: https://github.com/globaleaks/Tor2web/issues/168#issuecomment-118568263

This ticket is to make up a new version of RecommendedTBBVersion in a simplified format, containing always and only 2 versions:

• Stable one
• Alpha one