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;
Expand README, add CREATE EXTENSION
Add basic unit tests for "make installcheck"
Merge pull request #1 from intgr/master

Convert to PostgreSQL extension, update README, add tests