Adds a context to all JSON postmessage data. Without some sort of name spacing it is very easy for there to be a conflict between various postmessages flying around. We experienced a problem with a vimeo "ready" message being read by receiver.js and throwing an exception.

Fixed an issue where data.value could be undef, and data.value.src would throw an exception.
Adds version, a test and updates the spec
Merge branch 'hapyak-namespacing'