Added pushbullet agent, refactored JSONPath merging

Moved the JSONPath merging into a concern because it is now used by
two agents, I also added Agent#received_event_without_error? which
is used by the hipchat and pushbullet agents.
Moved working helpers into a concern, explained JsonPathOptionsOverwritable
Fixed typos, logging errors via Agent#error
Merge pull request #281 from dsander/pushbullet-agent

Added pushbullet agent, refactored JSONPath merging