Add testing on manager classes
Installation doc
Add documentation on bundle usage
Documentation link in README
Change doc structure
Document image usage
Twig extension documentation
Add info on unit tests
Add reference documentation on Manager classes
Improve phpdoc
Precision on file path construction documentation

Property visibility is often forgotten, this reminder will help.
Remove unused helper method
Throw exception when try to use a format that doesn't registered
Fix removing file when replaceFile method is used
Remove unused parameter
Fix tests after merge
Revert "Remove unused parameter" : for high configuration compatibility

This reverts commit 31cf23419a117e82976d9fd43197d9c034b09395.
Simplify test configuration