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

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Makefile & project skeleton based on http://manager.pgxn.org/howto

If this looks good then I will proceed to also convert collate.

I would also like to publish these on PGXN (http://www.pgxn.org/), but I don't know if it's appropriate to do it under my account. A META.json file also needs to be written for that.