Full Text Indexing Patch

Hi,

This patch adds the following to the FTI module:

* The ability to index more than one column in a table with a single
trigger.
* All uses of sprintf changed to snprintf to prevent users from crashing
Postgres.
* Error messages made more consistent
* Some changes made to bring it into line with coding requirements for
triggers specified in the docs.  (ie. check you're a trigger before casting
your context)
* The perl script that generate indices has been updated to support indexing
multiple columns in a table.
* Fairly well tested in our development environment indexing a food
database's brand and description fields.  The size of the fti index is
around 300,000 rows.
* All docs and examples upgraded.  This includes specifying more efficient
index usage that was specified before, better examples that don't produce
duplicates, etc.

This functionality makes fti greatly more useful!

Chris & Brett