Allow SLAssert methods to be called from non SLTest subclass methods

What do you think about this? I was planning on building a library of helper methods to build up the test cases, but unless they subclass SLTest, I can't call the assert methods. This doesn't seem right to me. 

This is my proposed solutions, but I wanted to get some thoughts.
I think this is a good point, though I find the phrasing a bit confusing. What do you think about

> @warning If you use an assertion inside a dispatch block--if your test case dispatches to the main queue, for instance--you must wrap the assertion in a try-catch block and re-throw the exception it generates (if any) outside the dispatch block. Otherwise, the tests will abort.