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.

.gitignore

@@ -37,6 +37,7 @@
 /examples/private
 /htmlcov/
 /sdist/
+/tags
 celerybeat-schedule
 parts/
 pip-delete-this-directory.txt