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.
(19864) num2bool match fix

This is a bit more heavy-handed than I might like, but it does appear to
do the right things:

* accepts numeric input appropriately, truncating floats
* matches string input against a regex, then coerces number-looking
  strings to int
* makes a best effort to coerce anything else to a string, then subjects
  it to the same treatment
* raises an error in the event of incorrect number of arguments or
  non-number-looking strings

I've also included some additional unit tests.
Cleanup per adrianthebo suggestions

* use Float() to process string arguments
* get rid of doubly nested arrays
* removing needless ternary operator
* improving error message handling
Merge pull request #139 from hakamadare/19864_num2bool_match_fix

(19864) num2bool match fix