Tidy up migrations to stop them breaking when the models change.

Conflicts:
	db/migrate/20120224230816_add_tenant_to_models.rb
	db/migrate/20120303112620_create_and_assign_default_tenant.rb
Add Rake task to create and assign first tenant, instead of the migration.

Conflicts:
	lib/tasks/spree_multi_tenant.rake
Remove require testing_support/env
Add tests for raketasks.
Backport changes from 1.2 branch. Fixes #3