Add node/edge sorting
Add sensible defaults for color and label
Use getNode/EdgeAttrib in getNodesWithAttr and getEdgesWithAttr

Also, use snapEquals() for greater consistency.
Strictify some equality comparisons
Add examples for K7, MST, toposort
Remove placeholder graph
Make "undefined attribute" error message more informative
Merge pull request #50 from Muon/master

Sort function, sensible defaults, equality fixes, examples