Improve metadata lookup (truncated query strings)

[stakats]

When looking up the following item, Zotero only generates a query string of "from:" "Guide from:" "of", perhaps because the first few pages of text are just a table of contents.

​http://www.sed.manchester.ac.uk/idpm/research/publications/wp/di/documents/di_wp05.pdf

[dstillman]

It currently only looks at the first two pages. It probably needs to be increased to at least 3.