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.
Edited example to include ValueWeightPostingSource

The js example now uses a Xapian.ValueWeigthPostingSource.
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.
Create Xapian::Query with Xapian::PostingSource

When a Xapian::Query uses a Xapian::PostingSource it should use the
relevant constructor that takes a Xapian::PostingSource object.
Fix XAPIAN_GLIB_AVAILABLE_IN for new XapianQuery symbols
Release reference to Xapian::PostingSource impl on finalize
Call constructed() parent method before initializing our own fields
Move heavy-lifting of slot property onto XapianValuePostingSource

It turns out that XapianValuePostingSource is not abstract. Move the
handling of the slot property in that class and make it instantiatable
too.
Add an example for simplesearch with pageRank
Add an accessor for XapianValuePostingSource:slot property

And use it in XapianValueWeightPostingSource.
Fix gtk-doc annotations

And expand the docs for some methods previously undocumented.
Merge pull request #15 from endlessm/sdk/2009

Implement ValueWeightPostingSource class bindings