Refactoring

Mostly cleaning up and adding comments though.

Most important additions:
- UMD
- sourceGetterFunction (as a setting)

New api:
```js
// settings
Sloth.Base.settings = Sloth.Base.extendFromDefaultSettings({
  sourceGetterFunction: function(modifier) {
    return [
      this.$element.attr("data-src").replace(/\/$/, ""),
      modifier
    ].join("/")
  }
});

// base — used to be 'new Sloth()'
new Sloth.Base();

// loading
Sloth.Base.load();

// subclasses
new Sloth.Inline();
new Sloth.Background();
```

Not sure about the subclasses and the settings. What do you guys think?