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.
Add non-intrusive support for AWS to Vagrantfile

This changeset adds support for AWS provisioning to
Vagrantfile. It won't get in the way of regular users
who want to just use Virtualbox.

The AWS provider will be required only if the user does:

vagrant up --provider=aws
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
Merge pull request #283 from racktear/master

Add non-intrusive support for AWS to Vagrantfile
Fixed an issue not symlinking the .env file

Also updated the .env.exmaple to match the default one as close as possible