Make shift-tab focus on the previous control

As it would when working with native form controls. Does not work in IE
yet, but so does regular tabbing at the moment.
Makes tabbing in/out of the control also work in IE

Follows tabindex attribute when using the tab / shift-tab keys through
the form.
Adds option so select items with enter key
Changes option to allow any specified key to be used for selection

As discussed.
Merge pull request #95 from Frique/enterkey_option

Adds option to select items with enter key
Merge pull request #94 from Frique/shift-tab

Tab and Shift-Tab key behavior improvement