hstore extension version screwup

When adding json support to hstore, I made a major blunder and added the
new functionality to the existing sql script instead of bumping the
version, renaming the script and adding an update script.

This was lazy and there's no real excuse, although I will note that it
was a mistake far too easy to make. Perhaps as a warning indicator we
should remove write permissions from these files.

Anyway, I have had some discussions with Dimitri, and the best idea
seems to be that we should do all the above, but in the update script
use conditional logic that only adds the functions if they aren't
already there and dependent on the extension. In the release notes we
should advise anyone who has loaded hstore to run 'ALTER EXTENSION
hstore UPDATE TO '1.2';"

The minor downside of this is that the upgrade script will depend on
plpgsql be available. We'll need to note that too, although I don't
recall the last time I came across a site that didn't have it loaded.

See attached for details of what's proposed.

cheers

andrew