`inject_on_load` default false

I think it's a good idea to change default value of configuration parameter `inject_on_load` to  `false`. In most cases developers don't need file to entity injection but if turned on it takes a lot of time and memory. Especially when you use Gaufrette to upload files to S3.

I've spend much time profiling to find what makes my app so slow. This pull request is to save time of other devs.