Enable num2bool to accept numeric input

Also ignore rspec fixtures directory
Reworked number-handling logic

No more coercing to String and regex matching.  Instead, we now coerce
to Integer at the beginning or raise an error if we cannot coerce to
Integer.

A consequence of this change is that the function will now accept
blatantly non-numeric strings as input, and return false.  This seems a
bit goofy to me, but it's how String#to_i works.  If we really don't
like this, then I'm open to suggestions.