Update Guava to v15.

This decreases the performance hit from checking of the incoming buffer
uses JSON (see #80).
Update Jackson JSON parser library to latest version.
Decide when source is first added what the payload format is.

Fixed #80 - switching data types is an uncommong enough thing that I'll
ask that you re-add the source if the format changes.
Beware of negative message lengths in corrupted binary payloads.