Moved Xml-based configuration to a new dll (Closes #84)
- New dll StructureMap.Configuration.Xml
  Once referenced, the user can call the same methods on the configuration
  expression, just that they are now extension methods
- All tests related to the xml config functionality has been moved
  to Structuremap.Xml.Testing: The structure, though, has been flattened
  and some parts of the DataMother have been duplicated, while some files
- StructureMap itself does not know about xml-based config, which is
  proven by the removed dependency to System.Xml

What is missing in this commit is setting up e.g. a Nuget that would bring
XmlConfig capabilities to StructureMap.