load hmac folder in simple_auth.rb
Middlware DSL & refactoring

With the new middleware dsl some major changes has occured.

First of all HMAC is no longer a Class and represent a Module now.
Under this new Module 2 Classes were created:
  - Middleware
  - Config

The middleware class is the interface to the rack api (#call method)
On initializing (when using 'use ...') a HMAC Config object is instantiated.
The instance variables of this config object (tolerance, stepsize, secret ...) is
populated via yielding the middleware block into the config object

Open TODOS:
  - refactor Middleware#request_data
  - add error handling to config object (wrong dsl directive should output nice error)