Fix Tornado's template path resolver for absolute Windows paths.

Ues a shim to patch tornado.template.Loader to use os.path.isabs() to
portably detect absolute paths. Fixes an issue where the path to
extended templates can't be resolved from a nested template that was
itself loaded with an absolute path. Includes a test.
Merge pull request #156 from igetgames/fix/tornado-win-Loader-resolve_path

Fix Tornado's template path resolver for absolute Windows paths.