Merge pull request #203 from njoyard/add-capabilities

Add codec and format capability helpers
Merge pull request #205 from njoyard/fix-tests

Fix bitrate arguments tests
Inherit from EventEmitter, deprecate onStart/onProgress/onCodecData
Deprecate callbacks in favor of 'end' and 'error' events
Update tests to evented API
Fix wrong mergeToFile callback chaining
Enable kill an renice runnning ffmpeg process
Add tests for .kill and .renice