Curried operator functions can help reduce the amount of validators

Idea originated from https://github.com/racker/ko-validation/pull/38#issuecomment-31648947

Ultimately, I want to remove unusual validators (like `minLength`, and all the `dependentFieldNumericValidator`) from ko-validation. With a few provided operators, these corner cases could be covered with very few code outside the library.

- [x] write tests for operators
- [x] write tests for `integerValueValidator` and kill `greaterThanValidator`
- [x]  write tests for `lengthValidator` (not the `maxLengthValidator`, which is already tested)

future PR:
- [ ] remove `greaterThanOrEqualToFieldValue` and `lessThanOrEqualToFieldValue` validators