Fixed DB insert function

Fixed insert func in db.mysql.php - was causing double table prefix when
using it.
Fixed registration page

Removed captcha.php - it will be later replaced by simplier example.
New array validation function add

Added Function: isset_array, which checks if an array has some keys, and
returns true. It is used in registration process and needed to check out
every field that user is submitting. Just check out Register module to
see how it works.
Minor style changes for DB exception errors