INVITE difficulties

[vodik]

For an internal tool, I've added basic b2bua support for INVITEs, so calls can be made between registered devices. However the implementation is lacking, and only barley works.

• Can't currently catch dialog closing events properly - the iterator doesn't end on BYE so it hangs.
• In expansion of the above, I can't capture CANCEL messages

Proxying is probably similarly painful but haven't tried it.

Musing on this, I think it might make sense to bake b2bua support into aiosip with a similar API to how proxying is done (not that I'm happy with the proxying API, but I got some ideas there as well).

[ovv]

Proxy API was changed with all the dialogs rework, I'll try to add an example next week.

[vodik]

Closing for #124