Add unit tests for the labels functions.
Check if prolog rules are defined for this project or its parents, and
if not default to the added Java implementations of LabelFunctons.
Before this commit, the Prolog rules engine was always invoked to check
wether a change can be submitted or not, even if no prolog rules were
defined.
Doing so should also make it easier to extract Prolog as a plugin
without losing any of its currently offered features (label functions
and default rules implementation).
The LabelFunction code is inspired by Saša Živkov's change Iffe1567,
adjusted to live directly in the enum.
Change-Id: I5e18b0d494be3f0423bb533ed84a63ea4b8a31df
c275089fc9