Let ObjectMapping can be used as a standalone library in cocoapods

Changes are quite simple:
- remove useless codes (some #import) of Network subspec from ObjectMapping.
- add a subspec named ObjectMappingSupport for ObjectMapping, old one uses global RestKit/Support.

Now others can use ``pod 'RestKit/ObjectMapping'`` in their Podfile.
And without restrictions from Network subspec, AFNetworking 2.0 can work together.