Implement _.bound

Based on https://gist.github.com/evilhackerdude/5783174

Thank you @raganwald for the `length` argument.

Questions:

- `_.bind` or native `bind`? I'd say `_.bind` for greater reach.
- Should obj[fname] be checked by `_.isFunction` and throw an exception if it isn't? I've seen a few similar checks in other functions.