Coding Style & clean-up

We had been using Drupal 8's TFA, which includes something much like this module, and wanted to see it work in D7.

Glad I found it -- did some cleaning up of the coding style, a few small changes like default values for the variable_get, removed a commented out dsm(), fixing whitespace, a fail-out if the 'tfa_enabled' flag is not set in the main TFA module.

For example, on my local site, I keep the TFA module enabled, but in settings.php I set the variable 'tfa_enabled' to FALSE, and I don't have to use TFA on my development box. This works really well, and I added a switch for that here.

Also, I set a default for the 'force_tfa_pages' variable to be 'user/*/security/tfa' because I was having trouble setting up the TFA unless this page was allowed.