Add $attributes param to asset function

This commit adds a second parameter $attributes to the following functions:

- AssetMaker::addJs
- AssetMaker::addCss
- AssetMaker::addRss

This is useful when dealing with javascript libraries like require.js, which depend on these HTML attributes to function properly.

It does not break the existing code!
Minor fixes
Merge pull request #98 from alxy/patch-1

Add $attributes param to asset function