Allow the SPA to accept & expose capabilities
Rename Conversation.data to Conversation.context for clarity.
Store SPA capabilities in the Conversation.

- Store the capabilities in the SPA wrapper.
- Construct Conversation using the SPA wrapper.
- Store SPA capabilities within the Conversation.
ChatApp stores received SPA capabilities.
Use model attributes to store capabilities.
Add a "supports" method to the Call model.
Updated CallControlsView to show/hide call move button.
Added initiate-move event

- added call move button event listener
- initiate-move event triggering & forwarding to the the worker and the SPA
updated TalkillaSPA to accept an options object
Fixed broken call to appPort#post
Fixed removal of explicit json serialization of offer payload.
Merge pull request #326 from ametaireau/call-move-support

Call move support