Implement EventTarget interface

[b-ma]

List of all events in the spec:

• BaseAudioContext::statechange
  • AudioContext::statechange
  • OfflineAudioContext::statechange (requires OfflineAudioContext::state is not updated when calling supend and resume orottier/web-audio-api-rs#416)
• AudioContext::sinkchange
• OfflineAudioContext::complete (requires Emit oncomplete event for OfflineAudioContext orottier/web-audio-api-rs#411)
• AudioScheduledSourceNode::ended
• AudioRenderCapacity::update (not wrapped yet)
• AudioWorkletNode::processorerror (not wrapped yet)
• ScriptProcessorNode::audioprocess (not implemented yet)

[b-ma]

Closing in favor of #28 and #122