Consolidate common code
Extract the common datetime parsing function
Make message_10 documentation examples testable
Promote nested classes to module-level classes.

Add aliases at the bottom for backwards compatibility.
Update documentation and tests to not use deprecated aliases.

I tested this by commenting out the aliases, fixing everything,
and then re-enabling the aliases.
Update documentation to reflect that nested clases are noi longer used.
Add tests for deprecated forms
Merge pull request #67 from TAXIIProject/reorganize_10

Reorganize message_10 module