Add an `appbundle` function to the builder DSL

This function takes advantage of an embedded copy of `appbundler`.
Appbundler reads a project’s Gemfile.lock and generates locked binstubs.
Ensure Builder DSL Ruby methods run under the embedded Ruby
Merge pull request #342 from opscode/schisamo/appbundle-dsl

Add an `appbundle` function to the builder DSL