Appbundle Chef, Berks and TK

Adds `appbundler`: https://github.com/opscode/appbundler which is a tool that generates binaries with locked dependencies from a ruby application's git repo. This allows us to follow the normal workflow (run `rspec` or whatever from the git checkout) for testing applications (which will include chef, berks and TK).

To that end, berks and tk are installed from git now. We still `gem install` them, but we don't pass the `-n PATH` option to force the gem's executables to install to the top level `bin/` directory. That executable is generated by appbundler now.

**NOTE:** We probably want to avoid pulling this in for `chef-client` until we test.
**TODO:** appbundler needs windows support. The build would probably work but it won't yet create the batch files you need to make the `chef-client` (and etc.) commands work.