Last verification round of all uses of Taxonomy meta -> account for possibility of `false` being returned

Also:
* Refactored form_row() method for readability and self-explanatory variable names
* Moved the one css rule which applies to the taxonomy edit page to it's own file, including .min file
Removed check for WP > 3.4 as plugin minimum is 3.5 anyway
Added extensive documentation about the new classes, how to use and the rationale behind them

Also:
- Increased priority for register_settings call
- Fixed small bug