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. 
If `conn.state == :chunked`, we should say `Chunked`. Also, maybe it is better to say `Send` instead of `Sent` because at this point we haven't sent yet?