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. 
```iex
iex(1)> :math.pow(256, 16)
3.402823669209385e3
```

That's how many entries we get. I would go with 4 or 5 bytes.