Use angular.isArray instead of Array.isArray.

Array.isArray is ECMAScript 5.1, and unsupported by older versions of IE. Rather than polyfilling the functionality, we should just use angulars built-in function.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray
Merge pull request #70 from voxtex/use-angular-is-array

Use angular.isArray instead of Array.isArray.