configurable term limit

Issue Summary

I've created a patch to configure the maximum amount of terms searched by the extractor. This is currently hard coded. Right now, if you have a vocabulary with more than 2000 terms you can't properly search for each term. This patch adds a configuration option on the extractor admin settings page and a variable in the extractor_simple.inc which defaults to the current 2000 but can be overriden. Furthermore, there is a small validation of the entered value to check that the term count is a numeric integer value.