add Dotenv.load!

`Dotenv.load!` is exactly the same as `Dotenv.load` except it fails fast and hard by raising
an instance of `Errno::ENOENT` if any of the filename arguments don't exist.

This changeset also includes some small refactoring to dotenv.rb since there was shared
functionality between the two methods (DRY and all that).
Merge pull request #27 from pje/load-bang

add Dotenv.load!