Fixed assertions for undefined logicals

Fixed bug where Assert:IsTrue(?) and Assert:IsFalse(?) both returned
true, these should return false and the null/undefinied value (?) is
neither true nor false.
Fixes #6
Remove noisy lines from error call stack

The callstack for a simple AssertionFailedError included a lot of lines
detailing the plumbing of OEUnit which makes it difficult to quickly
find which line number the actual error was thrown from - this is
particularly important when a lot of assertions are grouped inside a
test method. This change removes/filters the call stack of the error
objects to show calls from within the user's code - i.e. not OEUnit.