Basic Objective-C test code

As suggested in Issue #732, I have finally got around to adding code for testing Objective-C support.

This pull request includes everything related to giving Objective-C support a kick placed in tests/objc, along with a small README.md which may include information interesting to those who understand Emscripten better than I do.

_(Note: this is a rebase of pull request 1161 from master to incoming, plus an addition to AUTHORS.)_