Avoid defining Sprockets Sass Functions globally

Injecting functions directly into `::Sass::Script::Functions` is dangerous since its global and effects other `Sass::Engine` invocations that may not even be Sprockets related.

Doing this is still :cry:. I wish custom modules could be passed to the `Engine` options and extended for just one `Engine` instance.

``` ruby
module MyFunctions
  def my_url(url)
  end
end

::Sass::Engine.new(css, {
  :functions => MyFunctions
})
```

cc @nex3 @rafaelfranca

Closes #518.