Fixed issue that object ivariant was added into the wrong place in ObjectInvariant method.

Related Work Items: #13
Added support for property setter contracts for abstract classes and interfaces.

Related Work Items: #11
Added one more test case for adding postcondition for property in the interface.
Added support for older R# versions (8.0 and 8.1).
Merge https://github.com/SergeyTeplyakov/ReSharperContractExtensions