Add MANIFEST.in file so source distributions include necessary files

distutils by default expects LICENSE.txt, not LICENSE. It also needs to be
explicitly told about our example agent and associated files.
Automatically generate ChangeLog from git commits

We now have a ChangeLog automatically generated from the Git history, so users who do not
follow the proceedings in the Git repository itself can see what has changed between each
version. The ChangeLog is built whenever a source distribution archive is generated. It
can also be explicitly built by using "make ChangeLog".