Lots of dialog work and a back to back useragent example.

[vodik]

Two major changes with this pull request.

First, this hides create_dialog from the public API.

Instead of creating a dialog and then sending a SUBSCRIBE or REGISTER message on that dialog, we should couple dialog setup directly with the SIP transaction.

It makes for both a cleaner API that's more correct (okay, well, technically you could have always done the right thing with the old API, but you had to know what you're doing and do the appropriate bookkeeping yourself). If the transaction succeeds, you get a new dialog. See #69.

The tests and example code are both easier to follow with this change.

I also add a simple back to back useragent example. Its a variation of the subscription example that adds a third part - a location service.

• The registrar script starts a registrar that will accept registrations.

• The server script registers with the registrar and then waits for a SUBSCRIPTION, much like the subscription example

• The client asks for a subscription from the registrar. If a matching subscription is found, the registrar proxies the subscription to the particular server.

[vodik]

Oh, also carries a copy of #71 in order to make this work.

[codecov-io]

Codecov Report

Merging #72 into master will decrease coverage by 0.97%. The diff coverage is 62.06%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   71.45%   70.48%   -0.98%     
==========================================
  Files          15       15              
  Lines        1461     1484      +23     
  Branches      262      263       +1     
==========================================
+ Hits         1044     1046       +2     
- Misses        311      333      +22     
+ Partials      106      105       -1

Impacted Files	Coverage Δ
aiosip/dialog.py	75.83% <100%> (-0.84%)	:arrow_down:
aiosip/application.py	67.04% <100%> (ø)	:arrow_up:
aiosip/dialplan.py	86.66% <50%> (-3.81%)	:arrow_down:
aiosip/peers.py	67.01% <59.09%> (-2.39%)	:arrow_down:
aiosip/transaction.py	62.06% <0%> (-2.3%)	:arrow_down:

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[vodik]

Okay, going to go ahead and merge since I got a warm reception with #69