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. 
Heroku does:

    The value must be between 20 and 200 characters, and consist of ASCII letters, digits, or the characters +, /, =, and -. Invalid ids will be ignored and replaced with generated ones.