2.x port

- A 2to3- or 3to2-based automated port seems unlikely, so I split
  the code into separate directories, started with a 3to2 fix,
  and hacked on it until it worked with both 2.5 and 2.7. The
  tests are based on the 2.7 stdlib tests, not the 3.4 versions
  (because way too much other stuff changed in 3.x--in particular,
  the tests depend on what opcodes exist, how methods in classes
  work, etc., in ways that the actual code doesn't), so they may
  not be all that good.

- Also added a workaround for the setuptools bug described at
  http://www.eby-sarna.com/pipermail/peak/2010-May/003357.html.