Draw other people's cursors behind CodeMirror.

Set z-index of other cursors to 0 so they don't interfere with codemirror's click handlers.  Without this, if you click on/around somebody else's cursor, your cursor won't move since the click is going to the pre element instead of to CodeMirror.
Merge pull request #18 from mikelehen/master

Draw other people's cursors behind CodeMirror.