Refactoring in asset location wrappers
All asset location wrapper now extends (directly or undirectly) a new BaseLocationWrapper. The new base class contains common code shared by all location wrapper.
Added a new CdnLocationWrapper, allowing to process assets coming from CDNs: all relatives paths are replaced by absolute ones (only when minification or aggregation is enabled)
All wrappers have now an "active" property: all wrappers are active by default, except the new CdnLocationWrapper.
Fixed typos in log messages
Renamed "compression" by "minification"
Compression will be reserved for GZIP compression