Convert SLAssert{True,False} expressions to BOOL in a safe way (resolves #13)

Changes `BOOL result = (expression)` pattern in two places to
`BOOL result = !!(expression)`.  In the former code result will
be NO even if expression is non-nil if it just happens that the
first sizeof(BOOL) bytes of expression are zero.
Add unit test to ensure SLAssertTrue behaves nicely on expression with zero first byte (refs #13)

Adds a unit test to ensure that SLAssertTrue will not throw on
a non-nil expression that has a zero first byte.