Bug 948017 - Add sync option to subscribe API

Add optional ``sync`` parameter to basket subscribe API. Its purpose
is to tell basket to return a token on the subscribe, even if at
some future time basket is optimized to otherwise do all the subscribe
work in the background.

Notes:

* Not sure how to test this, since it doesn't actually change the behavior currently.
* Will need a corresponding update to basket-client, which I'll work on next.