`derive()` is a method.
hashValue is just a computed property.
Put the comment in the right place.
description is just a computed property.
Move ParseTree to the top level.
Merge pull request #10 from robrix/methods

Methods
Merge remote-tracking branch 'origin/master' into delay

Conflicts:
	Hammer/Derivation.swift
Use the leaf initializer.
How did I manage that?
Much cleaner derivative of alternation.