Use create_table instead of checking table existance separately

Note that for the timestamp migrator, table existance is still
checked separately, since separate knowledge is needed to determine
whether updating automatically from the integer migrator table should
be done.
Make pg_hstore_ops extension integrate with pg_array, pg_hstore, and pg_array_ops extensions

If the pg_array extension has been loaded, automatically convert
input arrays to PGArrays.  If the pg_hstore extension has been
loaded, automatically convert input hashes into HStores.  If the
pg_array_ops extension has been loaded, return ArrayOps if the
resulting SQL expression would return an array.

This makes certain usage of these methods much more friendly.