sass.compile should accept None as the include_paths parameter

Currently, passing None as the `include_paths` parameter to sass.compile fails with the error 'include_paths must be a sequence of strings, or a colon-separated string, not None'. This causes the sassc command-line interface to fail when there are no include paths specified on the command line, because it passes None in this situation.

This patch fixes sass.compile to treat include_paths=None as equivalent to an empty list - I think this is a more 'friendly' solution, rather than changing the behaviour of sassc.