support user specified rsync excludes.

This pull request is a simple solution for configurable rsync excludes requested in #140 and #152.

Instead of introducing a new plugin specific configuration option this change uses the already existing options hash of synced folders.
It allows to specify excludes for each defined synced folder in the Vagrantfile.
e.g.:

    config.vm.synced_folder '.', '/vagrant', :rsync_excludes => ['chef/tmp', 'tmp']