Created an Update method to check for uniqueness and add exclusions if needed.

Added 'update()' method. This method functions almost identically to 'save()', however if a 'rules' array is not passed to it, it automatically check's the model's rules for 'unique' rules. If any 'unique' rules are found, the ID of the entry being updated is appended to the rule to exclude it from validation while still checking for uniqueness amongst other entries.

See: http://laravel.com/docs/validation#rule-unique for more information on uniqueness exclusions.