XML parsing for fog providers

 Extraction of the XML parsing tools shared between a number of providers in
 the 'fog' gem. fog is the Ruby cloud services library, top to bottom:
 .
  *Collections provide a simplified interface, making clouds easier to work with
  and switch between.
 .
  *Requests allow power users to get the most out of the features of each
  individual cloud.
 .
  *Mocks make testing and integrating a breeze.