enable rsync__args option (and rsync__exclude)

This enables the [rsync__args](http://docs.vagrantup.com/v2/synced-folders/rsync.html) option, which I wanted to use to pass the "--files-from=" option.

My edits would have created a merge conflict with the very related @sawanoboly pull request #110. So, I guessed it would be easiest for everyone if I first merged in that PR and built on top of it.

The rsync__args option needed vagrant >= 1.5, so the first commit here updates Gemfile like [here](http://docs.vagrantup.com/v2/plugins/development-basics.html), resulting in similarities to pull request #101 (commit 73f6e01).

This PR should offer a variety of options for issue #76 'rsync ignore files'.