Advanced features API proposal

This contains proposed API changes to support DTMF, Transfer, Hold, Multiparty Calling, Stream Manipulation, and Provider-Specific Settings.
Transfer target protection option

Add a parameter to Call.transfer to turn on transfer target protection, as in RFC 5589 Section 7.2.
Remove providerConfig

Decided in meeting that it is best not to include this.
Change name of parameter in mute/unmute

Change parameter name from "target" to "targets" to clarify that multiple users may be given.
Merge pull request #10 from skapauan/patch-3

Merging 4 commits now included in the pull request per Nicole B.