Switch to xctool

[jspahrsummers]

Use xctool instead of xcodebuild, because dealing with the latter requires way too many hacks. Note that this means that xctool must be installed on the build machine.

I took this opportunity to significantly clean up the code. It actually looks kinda nice, if you squint and view it from a distance.

Also, bootstrap now automatically runs script/bootstrap for any submodule that has it. So there's that.

[jspahrsummers]

This currently disables unit tests for iOS, because attempting to run them in the simulator (at least on our CI server) results in the following error:

Failed to query the list of test cases in the test bundle: Error while parsing JSON: The data has been corrupted.: {
    stderr = "";
    stdout = "";
}

This seems related, but not identical, to a bunch of xctool issues regarding the simulator.

In any case, the missing iOS tests are still better than the broken state of master on Xcode 5.