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.

Require SSL and API key when subscribe sync=Y
Merge pull request #88 from dpoirier/bug-948017-add-sync-option-to-subscribe

Bug 948017 - Add sync option to subscribe API