Support providing the final release versions of intellij

This patch (and set of commits) provides functionality that allows the build-package process to use the -r (release mode) option to prepare a package of the latest 12.X released version.

It uses the jetbrains website to reliably determine the most recent release, and downloads it accordingly.

I have confirmed that the eap option still works, not using the -r flag still builds eap-style (so it is currently backwards compatible), and the release option does in fact download the release code.

I tested the install locally (actually replaced an EAP version with the last release), and the install seemed to work (and to my testing, it worked as expected).  I then installed the latest EAP on top of it, and again all seemed to work as directed.

Hopefully stepping through the commits in order will provide some confidence that this is a low impact patch and you will be amenable to including it within the trunk.

Thanks