Make an assetman.settings.load top-level function

This would just be a style improvement.  It would change usage from this:

``` python
assetman_settings = assetman.settings.Settings.load(settings.get("assetman_config"))
```

to this:

``` python
assetman_settings = assetman.settings.load(settings.get("assetman_config"))
```

I think that would be better.
