Allow client to change driver/provider using HTTP headers

[mfojtik]

Hi,

This patch will allow our Ruby client to pass a HTTP headers
and change driver on-the-fly.

Sample usage:

# ./deltacloudd -i mock

client=DeltaCloud.new('mockuser', 'mockpassword', 'http://localhost:3001/api')

ap client.realms
# => Mock realms

ap client.use_driver('ec2', :username => 'xxAMAZON_API_KEYxxx', :password => 
'xxSECRET_KEYxx').realms
# => EC2 realms

  -- Michal