curpath moved into iterator

  [breaking change]
Stop depending on crates.io "log" (rust "log" is not deprecated)
Moved get_next_dependency() into the interators
Major Rework and API change:

  * Target moved into the iterator.  set_target() replaced by an
    iterator parameter.  iter() renamed to dependencies_of().
  * Iterator keeps it's own list of satisfied nodes, starting as a
    copy of the depgraph's.  Does not modify the depgragh anymore.
    No more satisfying_iter() variant.
Tests needed strengthening
0.4.0