Remove DStarGraph

DStar is not currently implemented (see issue #2).
Merge branch 'mutable-changes' of github.com:gonum/graph
Removed mutable from concrete names
Attempt at better NewNode
Deleted a leading space in comments
Removed useless convenience constants
Up and removed VetMutableGraph
Fixed mutability assertion
Removed EmptyGraph
EdgeBetween->UndirectedEdge (same for EdgeTo->DirectedEdge)
Directed->Undirected in MutableGraph (oops)
Refactoring
Refactoring