Added OHHTTPStubs pods

See https://github.com/AliSoftware/OHHTTPStubs
Method to get posts from STRAppDotNetProxy

The test coverage is minimal, because it relies on the fact that the components of STRAppDotNetProxy are well tested themselves.

This is more of a black box test, where what is tested is the behaviour (are the callbacks called with the expected values) rather than the implementation (stub the components to return certain values, then test that those values are what the callbacks get called with)

Plus: small typo fix in STRNetworkManager