Peng/add test harness

This changeset attempts to simplify testing in the local dev environment by allowing tests to run in shell with `rake test`. Further CI support will build on this Rake script.

Hopefully it's obvious that you need to install Xcode to run the `xcodebuild` command. Additional instructions are in the [readme](https://github.com/3dna/nationbuilder-ios/tree/peng/add-test-harness#testing).

Also, environment config has been moved out of the source and into an Xcode build scheme file, so at the very least multiple scheme files can be used during testing if desired. Attempts to allow config vars to pass through xcodebuild are unsuccessful, for now.