Test examples

Adds examples as tests.

I can't get examples/06-graph/07-graphml.py working on python 2.6 (nor was in in clips/pattern/master ! nor were a few others I fixed up). The reason is that we use ElementTree.register_namespace which is not available in python 2.6 (as far as I can tell). That is, it looks to me like it's proper <=2.6 bug (in master).

Note: as such this fails travis... we should address (or else skip this example, I already skip a few due to HTTPErrors, but this is a proper error) before merging. And probably add a unit test for whatever it is that's hitting this.

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Any ideas? Maybe it's this http://stackoverflow.com/a/8116716/1240268, not sure.

If it's non-obvious I'm tempted to wrap this part of `graph/__init__` in a NotImplementedError and simply skip the 2.6 version of this test (this is the easiest "solution").