Provide variant testing framework to experiments.

[lmorchard]

Issue #366 introduced data models and server-side APIs for feature flipping flags & conditions. This needs to be passed along to experiments, so let's do that.

Probably the easiest way is to do this via messaging using nsIObserver. But, investigating that should be a part of this issue.

[lmorchard]

Getting add-ons to cooperate close to startup time is proving a bit of a challenge. Going to swap this one to 4/8 and see if I can't get #479 done instead

/cc @chuckharmston @6a68