Linking http-stubs to AeroGearTests build phase.

To test: 
1. First clean out you build folder in Xcode using ```CTRL+SHIFT+OPTION+CMD```
2. ```rm -rf AeroGearHttpTests/aerogear-ios-httpstub/```
3. ```git submodule init && git submodule update```
4. In Xcode now run the test using ```CMD+U```