DynamicChoiceStringParameter: a base class for dynamic lists in ForceScheduler

Provide a better mechanism for generating dynamic content for ForceScheduler, and
adjust the InheritBuildParameter to use this new base class.
ChoiceStringParameter.getChoices: use a member function instead of DynamicStringChoiceParameter
ChoiceStringParameter.getChoices: a bit more documentation
Merge branch 'DynamicParameter' of git://github.com/jaredgrubb/buildbot