checkLoaded should use getAttribute

When a script is asked for it's src property it returns the full path of the loaded script. 
If some script is aliased using a relative path, and also preloaded in the page (for example jQuery), it will be reloaded whenever the alias gets its first hit, since the checkLoaded has the full path version as loaded, and not the relative version.

This fixes that issue.
minified version with bugfix

This version adds the getAttribute whenever checkLoaded is used.
Merge pull request #3 from ivanrey/master

Bugfix for checkLoaded when alias are used with relative paths