Make Comparator yield abstract errors + add formatter

[Majkl578]

Would be nice to make Comparator yield errors like new ClassFinalized() or new ClassDeleted() instead of hardcoding strings + names.

On top of that, have a processor/formatter that can:
a) categorize the changes by namespace, type of change etc.,
b) write specific formats - pretty console, conservative xml, ugly yaml (yuck!) etc.

[Ocramius]

At least for the first version, each comparator will simply yield a Change entry with a string description produced by internal Comparator\BackwardsCompatibility components.

Later we might indeed have a better idea of the data structure to use to represent a change, but for now it is unlikely something we can abstract until a pattern emerges.