add plugin infrastructure for Buildbot

* add IPlugin interface that should be a base for all interfaces that can be
  delivered as a plugin (currently, it does not have any properties)
* add `buildbot.plugins.db` to load and check plugins
* add basic unit tests
adopt plugin system for www service
provide bundled change sources as plugins

* this should help to "unify" the way changes sources are defined
provide bundled schedulers as plugins
provide bundled build slaves as plugins
be explicit about wildcard import

* this allows to inject them into configuration file and access necessary bits
  in a bit shorter way
provide bundled steps as plugins
provide bundled status receivers as plugins
introduce 'util' category of plugins

* this will help to make master.cfg files using only core functinality to look
  much simpler
add current master.cfg examples to docs/examples

* it will be converted to use with plugins
* the diff between this version and the target version will help to see if it's
  worth it
add an outline for the plugins docs

* add relnotes
* update examples/sample-plugins.cfg to use plugins
* add user manual content