added task rake db:seed to execute multiple custom seeds

Added rake task to execute multiple custom seeds that can be used for different environments / development runtime

	Usage:
	$ rake db:seed:{FILENAME AT /db/seeds}

	Eg:
	$ rake db:seed:my-seeds


my-seeds.rb file 

- fixes problems with uservoice_subdomain @ https://github.com/catarse/catarse/issues/166
- creates default users