Fixes issue #179

Added tags file to gitignore. This is the index file created by ctags which is used by some text editors and is developer environment
specific.

Added the option to use :if syntax as syntactic sugar for guard.

Added the option to use :unless syntax which essentially behaves
as a negated guard.

Added a couple of tests for the :unless syntax since the logic was
slightly different.

Noticed a couple of tests were just returning true/false at
the end rather than checking for expectations.  Fixed a couple
of those; still some remaining.

Changed AASM#perform method to allowed?