changing FileNotFoundError to IOError, for python 3.2 compatibility
issue4 - used MultiMarker to visualize node colors in red black tree
multi marker color can be explicitly set in tree.
Merge pull request #6 from dahpgjgamgan/master

1. changing FileNotFoundError to IOError, for python 3.2 compatibility
2. use of multimarker in rbtree
3. allow to explicity specify multimarker color