Sort MediaFiles.

os.path.walk() uses os.listdir(). And os.listdir() returns arbitrary order of
files inside the directory. This commit sorts the MediaFiles by natural
ordering thus the files will be uploaded in the order.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
Merge pull request #6 from grandpaul/sortedfiles

Sort MediaFiles.