fixes #230, this.previous returns wrong index

Accordion's `this.previous`is returning wrong values.
Example (wrong behaviour): http://jsfiddle.net/BgGCN/1/

this.previous is returning the current open, instead of last opened.

New version (correct behaviour): http://jsfiddle.net/BgGCN/
Merge: Fix accordion's `this.previous`

Accordion's `this.previous` was returning wrong values.
Example (wrong behaviour): http://jsfiddle.net/9TnkF/

`this.previous` is returning the current open, instead of last opened.

New version (correct behaviour): http://jsfiddle.net/BgGCN/

fixes #230
closes #1255