[eyeglass] Optimize asset registration

[chriseppstein]

map-merge is slow for maps with many keys. This patch:

• Changes the asset importer to generate Sass code that invokes @include asset-register-all() instead of many calls to @include asset-register().
• Changes asset-register-all() to no longer delegate to asset-register() for each asset. Instead it passes all the assets to js where the url normalization is done in a single pass over the asset data structure.
• On non-windows machines, asset url normalization is completely skipped. This makes asset registration a single map-merge onto a map with low cardinality.