add GerritChangeFilter

I realize I never upstreamed this simple change filter I made two years ago.

Hoping this will help to understand how things are supposed to be bound together.

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.