Move the keydown event to `td` to enable enter to save in quick edit

For some reason the keydown event wasn't firing when it was attached to `input` and `select` elements in the quick edit form. By copying core's metho and attaching the keydown to `td`, Enter now works to save the inline form properly.
Merge pull request #57 from Automattic/inline-edit-enter

Move the keydown event to `td` to enable enter to save in quick edit