Update Zotero Preview pane

* Remove splitter
* Add explanation of how Zotero Preview pane works
* Limit filtering to citation format
  Also add support for “citation-format” attribute of CSL 1.0
* Remove redundant warning
  (already happens on this.refresh)
* Use "items" instead of "references"
* Clean up HTML a little
* Fix some JSHint warnings
Merge pull request #593 from rmzelle/update-csl-preview

Update cslpreview.xul