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. 
Except that it leaves a trailing slash. You probably want this:

    Enum.reduce(~w(foo bar baz), [], fn(i, acc) -> [acc, ?/, i] end)