Add a release or binary unit type

[xizhao]

Source code can be distributed and received in two ways, a binary/release or raw source code.

It might be useful to define build targets as a new type, and use the graph data to map backwards to source units (or another type, as units aren't 1-1 w/ build targets). This can model more complex repo scenarios (i.e. repos that build different types of binaries or releases, and finally minified or binary files!) Maybe this is a clearer way to approach #165 by treating releases to be "binary" versions of a core VCS-managed repo.

If you can do this, then imagine the unreal features you can build on top of it. The obvious one is to browse binaries, .dlls, jars, minified/obfuscated code or any other compiled format in repos like the original, decompiled code.