Make dialog iterable instead of relying on a default handler

[vodik]

This API lets me work with a dialog without having to explicitly register a callback for every message.

It also means that there's no more implicit 404 handling either, but I'm not convinced you want to do that inside a dialog. I think that makes sense for the dialplan, but once a dialog is established, we probably want every incoming message to be explicitly handled, and with awareness of the order they arrived in.

Related to #65, but I'm going to close that and do it in two parts. If we get this in, I think it might make sense to look at removing or simplifying QueueTransaction API, and then proper INVITE support.

[codecov-io]

Codecov Report

Merging #66 into master will increase coverage by 0.07%. The diff coverage is 84.61%.

@@            Coverage Diff            @@
##           master     #66      +/-   ##
=========================================
+ Coverage   71.22%   71.3%   +0.07%     
=========================================
  Files          15      15              
  Lines        1449    1460      +11     
  Branches      259     260       +1     
=========================================
+ Hits         1032    1041       +9     
- Misses        309     311       +2     
  Partials      108     108

Impacted Files	Coverage Δ
aiosip/dialplan.py	90.47% <100%> (+0.47%)	:arrow_up:
aiosip/dialog.py	77.25% <80%> (+0.02%)	:arrow_up:

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[vodik]

Okay, going to start merging stuff since I'm building PRs off of PRs. As I understand it, I'm not going to break anyone's deployment, so here we go!