Support Seq pairs for select list options.

See google group thread for context:
https://groups.google.com/forum/#!topic/play-framework/2U13MZ9fJEg

Since the select method itself accepts a Seq[(String,String)], we should let users pass in a value of this type explicitly. As it stands Maps are useless since there is no defined ordering, so have to resort to ListMap hackery as a workaround.