Convert to a PostgreSQL extension

Now it can be installed with: CREATE EXTENSION musicbrainz_unaccent;

Older Postgres versions continue to work, but have to install the old
way.

Old databases can be converted with:
 CREATE EXTENSION musicbrainz_unaccent FROM unpackaged;

Tested with PostgreSQL 9.2 and 8.4
Fix typos and update link
Expand README, add CREATE EXTENSION
Missing comment in unpackaged--1.0.0 file
Add basic unit tests for "make installcheck"