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.

tests/util/requireable/dummy.py

@@ -0,0 +1 @@
+foo = 'bar'