Add createAbsoluteSourceStamps argument to Nightly

This is based on pull request #758.

When multiple codebases are used and createAbsoluteSourceStamps is
enabled, it uses the last revision seen for each codebase without
changes when creating the buildset. This stops any changes from the time
the buildset it created and source step is run from being included in
the build.
Merge branch 'nightly-abs-source-stamps' of git://github.com/pepsiman/buildbot