Addition of elixir logger and request_id

Adding basic logger support to plug. re https://github.com/phoenixframework/phoenix/issues/235

Also adding a unique request id to each request header, response and Logger metadata.

@josevalim I wanted to make sure you were okay with the addition of two plugs/API before I tested and documented. 
Btw, do we need 16 bytes? Isn't that a lot? Also, do they need to be strong? (I would say they do not).