Move to async plus add modify & notify

[danjenkins]

• Added an example Dockerfile using Multistage build
• Added in Modify & Notify methods so that you can now handle transfers/renegotiations
• Moved to using async/await purely because it encourages users to use them instead of chaining promise after promise
• Moved everything to use the same formatting - no spaces inbetween end of statement and the ;
• Enabled Drachtio Outbound mode - encourages use of it - so much easier to scale with it!

Not tested yet - hence draft PR. If you don't object to the async/await changes then I'll go ahead of test against a webrtc softphone

[danjenkins]

Thanks @davehorton - seems like I still have a bug around my modify logic though!