Fix October 27 Hangouts API change

This mostly affects the push channel setup.

Fixes #31
Update _parse_sid_response tests
Remove channel initialization optimization

Combining the two post requests sometimes causes the channel to never
receive any events.
Fix JS decoding for unicode surrogate pairs

This issue was exposed by emoticons being encoded differently in
messages using the new API.
Fix for API change: PVT token required to init

Fixes #45
Fix image messages

In the "new" API the list of message segments is optional, so we need to
handle it being None for image messages.
Merge branch 'api-fix-3'