Fixing ProQuest search, widening ebrary's domain

ProQuest search results now include ebrary e-books in an extra tab.
Zotero scrapes were returning everything regardless of which tab was
visible, and these extra items caused the ProQuest translator to crash.

This fix changes the scrape results based on which tab is visible
(database results or e-books), excludes some invaild sidebar links from
the results, and calls the ebrary translator to handle ebrary e-book
results. The ebrary translator was updated to handle links that come
from ProQuest.

Ignore ebrary results when using the bookmarklet

Tab detection logic moved into a new function

New getSearchResults function to get items from the ProQuest search vs ebrary e-book tab.
(This behavior is all within ProQuest; no need to change the ebrary translator.)

Fulfilling change requests

Removing unnecessary xpathing
Merge pull request #792 from proquest/proquest-ebrary-fix

Fixing ProQuest search, widening ebrary's domain
[DOI] Skip items with no titles when constructing multiples
Unfortunately, still breaks with single items. No way for us to detect this without doing DOI lookups in detectWeb
fix PDF download for PLoS