get MIME types without strictly requiring python-magic

There are two forms of python-magic, one included in libmagic that is
default on Debian, and another Python wrapper for libmagic that is
called 'python-magic' on pypi.  Those both rely on the compiled binary
library libmagic.  For platforms without good package management, fallback
to using the built-in 'mimetypes' library if 'magic' is not available.

This supports OSX, Windows, and BSD.

setup.py

@@ -26,11 +26,10 @@ setup(name='fdroidserver',
                   'examples/opensc-fdroid.cfg',
                   'examples/fdroid-icon.png']),
       ],
-      install_requires=[
+      install_requires=[  # should include 'python-magic' but its not strictly required
           'mwclient',
           'paramiko',
           'Pillow',
-          'python-magic',
           'apache-libcloud >= 0.14.1',
           'pyasn1',
           'pyasn1-modules',