FIX Directory Separator management on Gaufrette

When using Gaufrette (or actually any other File System manager) we could not handle Directory Separator with the constant Directory Separator because target directory separator could be different from system directory separator.

Example: application running on windows storing data on Azure or Amazon
 - Windows : DIRECTORY_SEPARATOR = \
 - Amazon : DIRECTORY_SEPARATOR = /
 - Azure : DIRECTORY_SEPARATOR = /

Directory separator should be manage by Gaufrette Adapter.

+ FIX on `resolveUri` method (which, before that commit, didn't convert `\` by `/`