COMP: declare virtual method for windows build

Apparently not needed on linux/mac, but required
for windows to override the method.
COMP: remove python hooks from loadable module

Windows will not compile these hooks, and they are not needed
because the python shared libraries can be directly imported
with a line like:

import vtkvmtkCommonPython

Next step will be to make a python module that encapsulates all
the vmtk libraries.
Merge pull request #4 from pieper/windows-fixes

compile fixes