stubAccessor can have adverse side effects

Added a failing test to demonstrate an interesting issue with `stubAccessor`.

As far as I can tell this breaks `Batman.Property`'s source tracking, preventing any properties with a dependency on the stub from being invalidated.

```
PhantomJS 1.9.7 (Mac OS X) Batman.TestCase stubAccessor does not cause adverse effects FAILED
	Expected: Two-Face
	Got: Harvey Dent
	    at /Users/davidcornu/Development/batman/node_modules/qunitjs/qunit/qunit.js:534
	    at /Users/davidcornu/Development/batman/src/extras/testing/test_case.js:83
	    at /Users/davidcornu/Development/batman/tests/batman/testing/test_case_test.js:185
	    at /Users/davidcornu/Development/batman/node_modules/qunitjs/qunit/qunit.js:203
	    at /Users/davidcornu/Development/batman/node_modules/qunitjs/qunit/qunit.js:361
	    at process (/Users/davidcornu/Development/batman/node_modules/qunitjs/qunit/qunit.js:1453)
	    at /Users/davidcornu/Development/batman/node_modules/qunitjs/qunit/qunit.js:479
```