Allow generate directory name basing on the generated file name

For example, the filename generated by the $mapping->getNamer() service used uniqid() and it is "49f3c69b3b96fdcad5542875679a7806.jpg" 
I want to use directory name, which will propose directory structure for this file like "49/f3". In this case, the directory namer should be called later, after filename was generated by the namer
Merge pull request #354 from rodgermd/master

Allow generate directory name basing on the generated file name
Allow the same mapping to be used more than once
Throw a specific exception when a mapping is not found
Provide tests for the templating helper
Provide tests for the twig extension
Updated UPGRADE instructions