Fix drag tab passing URI's

Fixes #71.

A couple of things here:

Turns out that removing the hyphen in the event data 'text/url-list' makes the data pass through the event on Linux.  So I renamed the key.

After that, I removed the duplicate data being passed through the drag event.  'text/plain' and 'text/urilist' have the same data for regular file, and 'text/plain' is meaningless for panes that are not files, like 'atom://config'.  So the URI should be sufficient in all(?) cases.

I updated the spec to reflect the changes I made.