@fancyremarker => Modify garnered_find to accept multiple arguments

Modifies ```garnered_find``` to accept a splat to match the API of Mongoid's ```find``` . 

You can now call ```Artist.garnered_find('andy-warhol','pablo-picasso')``` or ```Artist.garnered_find(['andy-warhol','pablo-picasso'])```

I think this is a straightforward fix except for the following cases:

```ruby
Artist.find('andy-warhol')  # Returns single object
Artist.find(['andy-warhol'])  # Returns an array of a single object
```

In these cases, the return type should match what was passed in. I couldn't figure out a great way to figure out from *args what the original arguments were, thus: https://github.com/artsy/garner/pull/63/files#diff-617bcfaf854a9b15e531702bca4eabafR64

That has the effect of properly returning an array of one (containing the object), vs the object itself (matching ```find```).