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Rack refactor and Sinatra integration

Refactor to enable usage of AppSignal with any Rack framework and
integration with Sinatra to start this off.
Minor formatting fix
Ignore all log files
Merge pull request #102 from appsignal/sinatra

Rack refactor and Sinatra support
Move middleware to aggregator
Monitor http queue durations
Merge pull request #103 from appsignal/http_queue_duration

Monitor http queue durations
Add appsignal require to Sinatra integration
Bump to v0.7.0.alpha.1 [ci skip]