Establish release process with version numbers

[mathjazz]

We should release versions of Pontoon and provide information on what changed in which release.

Many of our 3rd party installations don't use the master branch, as we do at Mozillla. Following releases will make it easier for them to decide when to update, in particular when any security fixes will be shipped. It should also help with how to update and what to be aware of.

Having a clear overview of what changed when will also help in tracking development progress and announcing new features.