Fix segfault.

If the endpoint doesn't have a CLOCK_DOMAIN descriptor then the set/get clock_source commands would segfault.
Add messages to inform the user if a command won't complete.
Merge git://github.com/audioscience/avdecc-lib
Merge pull request #106 from pthedinger/master

Fixes for errors found in testing.
Merge remote-tracking branch 'asi/master' into staging
add development sequence diagram
Merge pull request #107 from andrew-elder/staging

add throw/catch to descriptor read operations
Merge pull request #108 from audioscience/staging

Staging
Windows build requirement is now Visual Studio 2013 or later