API Refactor ClassName into DBClassName
API Update PolymorphicForeignKey to use new CompositeDBField and DBClassName
CompositeDBField is now an interface to nested fields on an underlying dataobject, allowing field manipulation to be performed at the field and dataobject level without having to synchronise them manually.
CHANGE Given that Config::inst is an implementation of the singleton pattern itself, I've removed the extra call to singleton(). A side effect of this is that it gets around a possibly nasty circular reference with the dependency injector (which relies on the config object); in future, this dependency structure should really be structured from the DI directly.
MINOR Change singleton and strong_create to use dependency injector
BUGFIX: Provide default constructor values for classes (fixes issues when used in 'singleton' scenario during dev/build in particular)
MINOR Clear out injector state when resetting db schema during tests (a follow on from changing singleton() calls to use the injector underneath)
