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. 
This if and the one below could be broken into its own function, this one is already too big.