Modify HttpRequestTemplate and HttpResourceGroup to be DI friendly

To enable ribbon in Guice just add RibbonModule() when creating the injector (either via Guice or Governator).  Alternatively one can override the bindings for HttpResourceGroupFactory and HttpRequestTemplateFactory to override HttpResourceGroup and HttpRequestTemplate. 
A solution to support creation of different ResourceGroup using DI is to make RibbonResourceGroupFactory an abstract class with methods for creation of each type of ResourceGroup. Initially it will only have one method `newHttpResourceGroup()` but later can add new methods like `newUdpResourceGroup()`. The default implementation would just call the constructor of each type of ResourceGroup. Same with RibbonTransportFactory.

That way we don't need to create a complex class hierarchy with generics and the injectable is simplified.