Win32 fixes

* uses GVDLL when building for Win32. Visual Studio build create correct binaries.
* avoid creating file handles when not needed. Stops tests from failing on Win32.
* some refactoring - mostly pep8 related
* regenerate using swig 2.0.10
* correct usage of graphviz headers
* hard code tests to work on win32

There are some fixes which are _not_ portable. Released as is to gather inputs and suggestions on how to proceed.