Update clients.py to use PEP8-compatible function names

The original names can also be used for backwards compatibility.
Rename some more functions.

We don't need to make backward-compatiblity adjustments for these, since they
should have only been used internally.
Rename internal function
Make HttpClient inherit from Object
Ensure only new function names are documented.
Merge pull request #121 from TAXIIProject/pep8-function-names

Pep8 function names