Updated version number to 3.00
Updated string1.tut
Added class_use.tut

The analysis for this tutorial is quite complex, and so it is a good
example of how to go about resolving #15.
Updated involved_identifiers to use varargs

This means that any node with multiple entries (eg Assign.targets) can be
passed in directly, eg:

    TutorialNodeVisitor.involved_identifiers(*node.targets)
Updated to use new argument getter
Merge branch 'master' into tests_framework