Adds drag-drop sorting when voting.

Quick summary of changes:
* Adds jQuery UI base library, which includes a .sortable() function
* When a sort event happens, determines the desired new ordering,
cancels the actual sort event (to prevent the UI elements from actually
being rearranged), then reorders the selected values to match the
desired order
Improvements to ballot entry drag/drop support.

* Downloads jQuery UI from Cloudflare (since other resources are from
there)
* Rearranges “reason” textboxes as well as just the votes
* Better visual indications: gmail’s move/grab hand cursors, border for
placeholder box
* Sizes the placeholder row correctly to match the current ballot row
height (should account for fluid layout)
* Sorts out the numbering issues caused by the placeholder element in
some browsers
Merge pull request #57 from gunsch/master

Adds drag-drop sorting when voting. Fixes #44, #51.