Split invoker to invoker and invokerN

- Eliminate optional argument.
- Remove unnecessary check for `arguments.length`.
- Add unit tests.
i'm interested to hear what @drboolean and @begriffs think about this question. 
How do you compose with Either?

Mike's position: curry in the left (message) and wait for the value to be passed in, e.g.
`R.compose(map(something), Either('someFn failed!'), someFn)`