JayTools: Fix for issue #3

- Fixed issue #3 which prevented the target from being assigned on existing modifiers if the mirror was not at the top of the stack.

- Fixed a typo or two in the tooltip.

- Removed @classmethod that made Add Target inactive if no object was selected.

Close issue #3 https://github.com/CGCookie/script-bakery/issues/3

Signed-off-by: Jonathan Williamson <jonathan@cgcookie.com>