Changes the twig loader in the MessageRenderer class.

This commit changes the twig loader from an \Twig_Loader_Array to a \Twig_Loader_Chain.

There are new helper methods to add new loaders.
Adds new loader argument to lexik_mailer.message_renderer service.

Adds new argument to the lexik_mailer.message_renderer service to allow the chain loading of the filesystem loader which will allow the rendering of filesystem templates.
Adds simpler chain loader implemention based on discussion in #10.
Merge pull request #10 from mathewpeterson/master

Adds ability to use twig templates inside messages