new method for Ink.Util.Array

I've been making my own array.unique() functions since I started using Ink, so maybe it's useful to have it on its core.

This works in a very simple way:
```
> Ink.Util.Array.unique([1,2,2,2,2,3])
[1, 2, 3]
```

due to its use of the Ink.Util.Array.convert method it works great with strings too
```
> Ink.Util.Array.unique('aasdasdasdasd')
["a", "s", "d"]
```

I'm also using Ink.Util.Array.filter so it uses the browser's js api for array.filter if available.

good point. Fixed. added a slower fallback when indexOf is missing.