Cache warmup

This PR introduces Interface change (new method collect) and, therefore, major version should be bumped if accepted.

It resolves problem https://github.com/RWOverdijk/AssetManager/issues/40 and it's a set forward solving https://github.com/RWOverdijk/AssetManager/issues/131

Please let me know if you have any concerns. 
Lines 79 to 82 should likely be abstracted away. The output at line 81 should go after the execution actually happened.