change head.load api to pass attributes to loaded dom element

As mentioned in https://github.com/headjs/headjs/issues/320 I would like to load a CSS file with media="print" attribute.
This PR is inspired by @robert-hoffman comments in https://github.com/headjs/headjs/issues/262#issuecomment-26493353 and in https://github.com/headjs/headjs/issues/267#issuecomment-27417241

It is now possible to add options to an asset. Implemented are additional attributes to the created DOM elements (link or script).

```
head.load({
           mylabel: "somescriptPath",
            options: {
                attributes: {
                    "data-main": "loaded1"
                }
            }
        },
       callback
);
```

I could not find any script to create the dist files, is there one?