Added Posting Source

Abstract class that allows external sources of postings to
affect results returned by queries.
Added Value Posting Source Class

Abstract class for a posting source that can generate
weights from data located in a value slot in database
entries.
Added Value Weight Posting Source Class

A posting source that reads value weights from a value slot.
Added posting source info to /doc files.
Added new_from_posting_source wrapper to Query.

Allows a new query to be created from a posting source.
Also added posting source to simplesearch script for databases
that include a posting source.