Add support for types on methods, change rule name w conflict

Now, you can specify a return type on a method using syntax like Mirah's.

    def foo(): void { puts 1 }

This also changes the Token rule to AToken to get around a conflicting name from BaseParser that's currently causing compile errors.

README.md

@@ -166,7 +166,7 @@ this parser, or the first.
 // this parser will turn into: `class Calculator < BaseParser; ...`
 parser Calculator {
     // defining a mirah method
-    def self.main(args:String) {
+    def self.main(args:String): void {
         ast = Calculator.new.parse(args[0])
         puts Interpreter.new.parse(ast)
     }