Added functionality to retrieve into content scripts the full or

partial text-only of a pdf, using existing find text extraction. This addresses issue #4137. However, the implementation is altered slightly from the requirements listed in issue #4137 in that a return event does not occur immediately in all cases in order for the implementation to be entirely event-driven and for the users of the functionality to be able to initiate behavior at any time (including before pdf.js is fully initialized). The behavior is detailed in a document (Full or Partial Text Extraction.doc) for users of this functionality that will be made available, but it seems there is no way to attach a file here except for PNG, GIF, and JPG images.