Buildout2 compatibility

I added a try/except and one now-optional kwarg to the monkeypatched _get_dist() method to make buildout-versions compatible with buildout 2.0.0b1.

TODO what I haven't done yet: change version number in setup.py and add release date in docs/changes.txt.

The tests still run, apart from getting a "killed" somewhere out of the "use.txt" manuel doctest. But I got that error also before I made any changes.

I started preparing another branch with some temporary changed to the bootstrap and buildout config to make it easy to test it with buildout2.0 directly, but I ran into z3c.recipe.scripts and zc.recipe.testrunner problems there. I did test it in an actual buildout 2.0-using buildout of mine and it works fine there.