allow attribute data-baseurl to set application baseUrl

Look for a data-baseurl attribute to set the baseUrl. Will override
data-main reference to baseUrl. Useful when require.js is included into
application startpoint and the application intend to do dynamic loading
of scripts.

This allows for a smooth way to handle the baseUrl based on different environments.

Example with require.js bundled in application.js
```<script src="//domain.s3.amazonaws.com/scripts/application.js" data-baseurl="//domain.s3.amazonaws.com/scripts/"></script>```