Util.operators: allow variadic methods to work with arrays

Variadic util operators, such as _.add(), can now work with argument arrays.

e.g. `_.add([1,2,3]) === 6`
Perhaps something like
`var numbers = arguments.length >= 2 ? arguments : arguments[0]`
or 
`var numbers = _.isArray(arguments[0]) ? arguments[0] : arguments`

would do.

Although I preferred the `_.flatten()` as it would allow you to mix numbers and arrays of numbers e.g.:
`_.gt([3, 2], 1)`