Make AssetScanner::find() able to return relative paths.

Make the resolve() method protected as it won't be needed soon, and make
find() able to return relative paths. This solves #207 where
raw asset urls are not found on the correct paths.

I've also made some minor changes to dynamic build files and raw assets.
When using dynamic build files, you should always properly prefix your
components, as they cannot be correctly located when outputting raw
tags.

Fixes #207