add GerritChangeFilter

Its not very clear for people how to use gerritchangesource to filter
gerrit change events, and create builds.

add GerritChangeFilter, which is a simple general purpose filter, which
manage most of the changefilters needs

doc + tests

and some whitespace fixing in the doc.

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
gerritfilterbranch: more factorization

This comes at the prize of a little bit of complexity

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
gerritchangesource: fix grouping for Nightly schedulers

for nightly and post-commit style schedulers,
you really need to have the branch to be the real gerrit branch
or the list of changes in the UI will be incorrect

This is a simple way of implementing it, probably by separating the
change grouping to the notion of change branch

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
gerritchangesource: test branch is branch for event type != changeset-created

and some cleanup of failUnless* API usage

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
Merge branch 'gerrit_change_filter' of github.com:tardyp/buildbot
fix pyflakes/pep8