Add index, first, and last to each helper
Fix syntax
Pass index as an int
Add a __len__ method to Scope objects
Don't bother converting index to an int (enumerate will always produce an int)
Add tests for first and last in each helper, based on https://github.com/wycats/handlebars.js/blob/fcec69ae2c838a9df7a456557b57671a80d1732b/spec/builtins.js#L112