Add package for universal graph utilities.

Move existing files accordingly.
Introduce base classes for graphs.

These classes are designed to provide basic functionalities for
inheriting classes.
Simpify control flow graph.

The control flow graph extends 'AbstractTwoWayGraph' now. Adapt all
dependent classes to work with the new interface.
Simplify control dependece graph.

The control dependence graph extends 'AbstractGraph' now. Adapt all
dependent classes to work with the new interface.
Merge pull request #20 from a0x77n/graphutils

Graphutils