Use bundler to manage vendored dependencies.

The rake init task was quite brittle, namely it didn't work
for me at all. Furthermore, it duplicates an awful lot of Bundler's
functionality. However we most likely *don't* want Bundler to become a
runtime dependency and we still want access to the gem sources for
packaging purposes. For that reason the new init rake task uses bundler
in standalone (no bundler needed at runtime) and installs to the vendor
path.

This patch may also improve startup time ever so slightly by reducing the number of calls
to File.expand_path from 6 down to 3.