Add APM support

• The signalfx-forwarder monitor is now configured and enabled by default.

Add Heroku Buildpack for SignalFx Agent

Add heroku buildpack for signalfx agent (#1)

* Add basic functionality

* Further enhancements to buildpack

 - Use envvar references in the agent config file
 - Allow the app to specify a config file at signalfx/agent.yaml
 - Don't do ingest/api default config in startup script (let agent do it)
 - Set bundle dir
 - Don't run in run dyno

* Add global dimensions to default config

* Optionally write logs to a file

* Update README

* Update default agent version

* Address feedback