Refactor stubbing helper method in base test case

Refactor into a helper that returns the request
dsl object, and a wrapping helper that replaces
the original and returns the response dsl object.
Allow base test stubbing helper to use any client

This allows creating HTTP stubs for test client instances.
Improve core client test case's Nocilla integration
Add OCMock testing library for test target
Mitigate andReturn DSL conflict between Nocilla and OCMock

OCMock's .andReturn will no longer work. Use
.andDo or -andReturn.
Clean up core client test case
Merge pull request #23 from 3dna/peng/test-infrastructure-1

Expand test infrastructure #1

Refactor Nocilla stubbing helper. Add and
integrate OCMock. Includes other minor test case
cleanup.