Minor bug fixes

* `tl.pg.init()` pulls in the default preferences if the user doesn't pass in a preferences object. This fixes #8.

* `open()` and `close()` now check to see whether pageguide is already open or closed before firing (fixing #13). Open state is stored inside PageGuide -- this way, we don't have to hit jQuery to check whether the body has the `tlypageguide_open` class.

* The "close guide »" in the open/close toggle tab didn't actually do anything because the close event binding's context was restricted to the message area. I expanded the context to include `.tlypageguide_toggle`.