Modified and extended build system to support wrapping in Python and more.

The new cmake file enables the following options:
* Wrap the library code in python (defaults to VTK_WRAP_PYTHON)
* Build dynamic vs static library (defaults to VTK_BUILD_SHARED_LIBS)
* Build the demo (defaults to YES)
* Build the demo as app (OSX only, defaults to OFF so that the examples
work)
Fix compiler warning for hiding an overloaded virtual function SetInput.
Enable loading extensions again on OS X.

This was disabled because of a bug in Snow Leopard, but it should/may
be fixed in later versions. At least it works on Mountain Lion with
Intel HD4000 graphics card.