Don't defer binding to the tab drop event

As of atom/tabs pull request #89
(https://github.com/atom/tabs/pull/89), the tab-bar doesn’t
`stopPropagation` of its drop event, allowing us to bind to it on the
pane.
Test dropping a tab into the pane
Merge pull request #9 from ahuth/cleanup-tab-drop-event

Cleanup tab drop event