Allow client to fetch drivers and regions

[mfojtik]

Hi,

This patch add two new methods to DeltaCloud class (client):

* DeltaCloud::connect(api_url, { :driver => '', :username => ''...}, &block)
  This will connect to API without credentials (and use HTTP header)

* DeltaCloud::drivers(api_url)
  This will return all supported drivers provided by API and also regions
  supported by each driver (EC2 for now as an example)


  -- Michal