Add a version number to bootstrap.py

Fixes #30
Eternal vigilance by eagle-eyed repository contributors?

More seriously, I don't think bootstrap.py should track zc.buildout's version number -- if a new buildout comes out with no bootstrap changes, there's no point in artificially incrementing the number.

What should happen is that people should change the version number in boostrap.py when they make other bootstrap.py changes.  I could add a test that fails whenever people change bootstrap.py in any way (compute sha256 checksum, compare with known version) and raises an exception with a message that tells them to:

1. increment the version number in bootstrap.py
2. copy and paste the new checksum into the test

How does that sound?