Bug 973992 - Support experiments add-ons; r=Unfocused

A new type of add-on is introduced: experiments.

Experiments are add-ons that conduct data experiments. They are deployed
to compatible clients and collect additional information about the
client.

--HG--
extra : rebase_source : 98a612933978b3e4bcfb83da9f6a9782c62dc0da
extra : amend_source : 9347a56bc3034c7ea1e50918278c6b9cc1270521