Email Support

Hey 
    I made it thread safe however to go form here I may need a bit of input on how to properly integrate it into the revel framework. For instance you mentioned instantiating the mailers as singletons. I would also like to setup a a framework for setting mailer defaults but how they are right now I am not sure I can do that either.

also related to issue #236 but that was not my issue so I could not attach to it.