Allow procs for label and value methods in grouped collection selects.

This is one solution to issue #623. I'm not to happy about monkey patching Rails though, are there better solutions?

This patch will not be needed when SimpleForm is used with Rails >= 4.0 thanks to this: https://github.com/rails/rails/commit/9035324367526af0300477a58b6d3efc15d1a5a8.