Adds a partly-implemented derivative function.

Concatenation and reduction are not handled, nor is memoizing. Recursive parsers are therefore not yet supported.
Merges in the is-equal predicate.

The is-equal predicate is required to test derivatives correctly.

Conflicts:
	Hammer.xcodeproj/project.pbxproj
Better implementations of -isEqual: for terminal parsers, such that the equality predicate can be used directly in -[HammerParser isEqual:] if so desired.
Implements the derivative as a visitor.

Believed not to handle recursion correctly at the moment.
Concatenation parsers export their child parsers.
Uses the parser’s children directly when determining their nullability and parse trees.
Monochrome Industries is no more.
README formatting for editor sanity.
Grammar ideas.
Ignore checkouts, etc.
Merge pull request #2 from robrix/feature/derivative-parser-visitor

Derivative parser visitor