Net::HTTP instrumentation
Merge pull request #130 from appsignal/net_http_instrumentation

Net::HTTP instrumentation
Use Appsignal.add_exception in integrations

This method is resilient against missing current transactions plus
this removes some duplication.
Merge pull request #133 from appsignal/add_exception

Use Appsignal.add_exception in integrations
Require active_support/notifications
Debug log when initializing Rack middlewares
Merge branch 'master' into develop
Make Appsignal#add_exception more consistent with Appsignal#send_exception
Use transactions without thread local
Delete transaction from list on complete! in ensure