add support for relative imports

per [pep 328](http://www.python.org/dev/peps/pep-0328/), relative imports add the ability to alter an import based on a "level" counter, which syntactically is a number of dots.   With these changes I've managed to produce successful graphs for parts of SQLAlchemy (which uses all relative imports) without any errors.  I've updated the tests though there should probably be some tests that test specifically the path resolution against the "sample-tree" package.