Add FlowToggle support.

Relays are now called "toggles" (since they might not
be relays in all cases) and are now first-class resources. 

Part of issue #184.
Minimal Controller admin.

Part of issue #184.
api: Scope URLs with version number.

Default to /v1 for now.
api: Update status response.
stats: Fix sessions_count.

Closes #201.