Initial Proxy spike

This will probably miss something important, like tests and could also
be a lot less hackish if there could be a way to get the current configuration
everywhere in Webmachine, one way could be to disallow this here:

``` ruby
MyApp = Webmachine::Application.new do |app|
  foo
end
```

and instead force users to subclass Webmachine::Application like this
so we could set the configuration to a class variable:

``` ruby
class MyApp < Webmachine::Application
end
```