Topological sort of a graph, allowing better console (and other) outputs.

Uses algorithm borrowed from networkx graph library to sort a graph in
topological order. The method is only used by output plugins, as
internal plumbery does not really care about the node order.

Also includes a bonobo.util.python.require function that helps importing
thing in a package-less context, or when there are conflict with site
package names.

bonobo/execution/base.py

@@ -87,7 +87,6 @@ class LoopingExecutionContext(Wrapper):
         finally:
             self._stopped = True
 
-
     def handle_error(self, exc, trace):
         return print_error(exc, trace, context=self.wrapped)