std.unittests for (final?) review [Update]

[Jonathan M Davis]

Okay, here's the new code and its documentation: http://is.gd/ktURa

I followed Andrei's suggestion and merged most of the functions into a highly 
flexible assertPred. I also renamed the functions as suggested and attempted to 
fully document everything with fully functional examples instead of examples 
using types or functions which don't actually exist.

So, now there's just assertThrown, assertNotThrown, collectExceptionMsg, and 
assertPred (though there are eight different overloads of assertPred). So, review 
away.

From the sounds of it, if this code gets voted in, it'll be going into 
std.exception.

- Jonathan M Davis