fix #669 rename concerto_configs group column to category

I've tested this change in sqlite3, mysql, and postgresql.  There's now a `database.yml.postgresql` example file, similar to the one for mysql-- and the gem for postgresql has been added to the Gemfile also.  I've also run rake test and all is well.

If someone can merge this when they're satisfied with it, that would be great.  Thanks.  This is for #669.