Adjust for changes in python 3.4 AST datastructures

In py34, a FunctionDef object changed its underlying datastructures.

FunctionDef.args.kwarg.arg changed from a string to an instance of
_ast.arg. The same thing seems to have happened for the *.args and
*.vararg fields.

This commit adds a small method to the visit_functiondef method of
the FunctionArgNamesCheck class which safely extracts the actual
argument name.

This addresses issue #10