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. 
Use `byte_size` which is much faster (`O(1)`) as we guarantee only ascii characters are in the string. Also, no need to ask for it twice, you can write `byte_size(s) in 20..200`.