Add support for subscriptions using sip.js >0.15.7

[confususs]

Version

0.1.10-beta.3

File / Feature

For the upgrade from SIP.js 0.15.6 > 0.15.7 the EventEmitter related functions on Subscriber are removed. Think of .on('failure', message => retrySubscriptionAfterXTime(message))or.removeAllEventListeners()`.

Figure out how retrying subscriptions should be done in 0.15.7.

Expected behaviour

WebphoneLib supports 0.15.7

Actual behaviour

WebphoneLib subscriptions don't work anymore in 0.15.7