replace SenTestingKit with XCTest

opening this pull request to begin discussion of moving away from SenTestingKit, Jenkins, and OCUnit

as checked in, all tests pass on both OS X and iOS using XCTest and running as bots on builds.karelia.com

with this we can safely remove — though I haven’t yet — everything Sam added for Jenkins: OCUnit2JUnit as a submodule, the test script, etc. Xcode Server provides all of that for free. I strongly suggest we make the switch.