New API calls to get only directories, rename or delete a file

1) Added a new GetDirList method. Compared to the GetFileList call (which returns all files and their full path from a given storage group) the GetDirList returns only the directories. Useful if one wants to show the folder structure only.

Example: http://BackendServerIP:6544/Content/GetDirList?StorageGroup=Fanart

2) Added new DeleteFile method. The storage group and the full file name must be specified.

Example: http://BackendServerIP:6544/Content/DeleteFile?StorageGroup=Images&FileName=path/filename

3) Added new RenameFile method. The storage group and the full file name must be specified. The NewFile value must only be the new name of the file. No path or whatsoever. So the renamed file will be in the same folder. The renaming will not work if a file already exists with the given name.

Example: http://BackendServerIP:6544/Content/RenameFile?StorageGroup=Images&FileName=path/filename&NewFile=newname

This includes the PR #35, because it was messed up.
Commit 5678eddc9d5067ab8db6bc41149e1bc70f7f6b2c can be ignored.