Fixed issue when scanning at the root of the classpath
Improved the cache system
A new extra has been added (dandelion-ehcache), allowing to plug-in EhCache as a asset caching system. The cache can be configured thanks to 2 new configuration properties (cache manager and cache configuration). It fixes dandelion/issues#192.
The SPI and built-in service provider has been updated (no more check if the key exist before trying to retrieve the cached content)