Source gem

This PR makes a source gem for `epf`.

Details that might need reviewing / approving:
* @ghempton is listed as sole author
* no gem email
* MIT license

Source gem is called `epf-source`. Would have been better to name it with the
`ember-` prefix to be in line of other ember-* gems out there, but `epf`
itself goes without the `ember-*` prefix, so, well, `epf-source` it is.