win32 build fixes:

* uses GVDLL when building for Win32
* 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
* Convert "doctest" style tests to nosetests
Merge pull request #8 from mindw/win32_fixes

Win32 fixes