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. 
I think cowboy will simply send it as `x-request-id`. There is no leading http. Also you probably want to pattern match on the result of `Dict.fetch/2` and call generate_request_id only in cases none was given. Finally, we need to ensure that, if we receive a request id externally, it validates to a given format.