rvoip 0.2.0

The 0.2.0 release is a beta candidate release for the SIP series crates. The entire developer API surface has been refactored for simplicity for the rvoip-sip crate (the SIP/RTP) library, I will try to minimize changes to existing functions and will be focused on finding and filling feature gaps and working on interoperability, performance, and reliability. If you want to build SIP apps, focus on the rvoip-sip crate and its dependencies.