Adds OCUnit parsing for more reliable exit codes

We're trying to use ios-sim to run tests in the iOS simulator on our continuous integration server. We ran into a problem where ios-sim always returned a failing exit code because it wasn't expecting the simulated app to exit after the tests ran and treated that as an error. These changes add a new "--unit-testing" parameter that turns on parsing of the simulator's output looking for OCUnit's end of a test suite message to determine whether the tests passed or failed. If any tests failed, then we return an error code on exit. If all tests pass, we return 0 on exit.

Sending a pull request in case other users would find this functionality helpful. This is my first pull request, so please let me know if it's not appropriate for the project or if I should set it up differently.