Rework

- rework Guardfile related classes (Evaluator, Reevaluator, Generator, Dsl, Cli)
- deprecate a few long names instead of contextually obvious shorter ones (e.g. evaluate_guardfile -> evaluate)
-  better error handling (between Dsl and Cli)
- merged Plugin classes (Base, Hooker, Plugin) since Guard:Guard has been removed
- start removing redundant namespace parts (e.g. `::Guard`)
- pass strings to instance_double as a workaround until a newer RSpec is released which can place expectations on const doubles