Shorthand method access. Related: #75, #76

It's smart to cache the methods, but I would still use the shorthand way to access them. jQuery [does the same thing](https://github.com/jquery/jquery/blob/master/src/core.js#L27). It's surely not much slower and it only has to be done once since they are cached. Also, arrays have an `indexOf` method and strings have a `slice` method so I renamed the variables to be more specific:

So instead of this:
```
  var _slice = Array.prototype.slice;
  var _indexOf = String.prototype.indexOf;
```

It would be this:
```
  var arraySlice = [].slice;
  var stringIndexOf = ''.indexOf;
```