Remove skipping of elements in sphinx documentation
Beginning to work on libtaxii.common module.

This module will reduce the overlap between messages_10 and messages_11
where possible.
Promote nested classes to module-level classes.

Add aliases at the bottom of the file for backwards compatibility.
The diff looks pretty awful, but you can use -w or ?w=0 to see the net changes,
which are pretty small.
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Update documentation and tests to not use nested-class aliases.

I did this by commenting out the aliases, fixing all the tests (both nose
tests and sphinx doctests), and then uncommenting the aliases.
Add note to update documentation once all nested classes have been removed
Merge pull request #66 from TAXIIProject/reorganize

Reorganize message_11 module