Start building firefox with buck.

There are some differences in the way that buck and crazy fun build things,
so this is just a first pass. Notable things that are missing:

* The actual XPI.
* The path to the preferences files has changed.
* No C++ components.

Essentially, this is just the java code and the preferences file.
Bump .buckversion

The buck build now relies on the "selenium" branch from Simon's repo. Clone from:

https://github.com/shs96c/buck.git

And switch to the remote branch "selenium". This contains the additional features
that our buck build depends on.

At some point, the main buck repo will implement a plugin API. At that point,
we'll switch to the main buck tree.
Build firefox xpts using buck