#35 Natural Filename Sorting

As per feature request 2951477, natural filename sorting is preferred by
most humans.
This patch uses strnatcasecmp (for case insensitivity) from the
suggested Natural Order
String Comparison (http://sourceforge.net/projects/natsort/) which is
ZLIb licensed and compatible with GPL.
However this doesn't help if a client subsequently resorts filenames
itself...
New source file was not added to make
Merge branch '#35-natural-sort' of github.com:Darkvater/minidlna into #35-natural-sort

Conflicts:
	Makefile.am
	upnpsoap.c
Merge branch '#35-natural-sort' into features

Conflicts:
	Makefile.am