package header shold be the full filename + ext

this fixes a bug in using `(package-buffer-info)`
Merge pull request #28 from milkypostman/master

package header should be the full filename + ext
Documentation added for :ensure keyword.
Added documentation to use-package macro.
Merge pull request #37 from phillord/ensure-doc

Ensure doc
Validate keywords. Error if any keyword is unrecognized.

Conflicts:
	use-package.el