Include license=MIT key in setup.py

Author: xolox

setup.py

@@ -66,6 +66,7 @@ def get_absolute_path(*args):
       url='https://github.com/xolox/python-rsync-system-backup',
       author="Peter Odding",
       author_email='peter@peterodding.com',
+      license='MIT',
       packages=find_packages(),
       entry_points=dict(console_scripts=[
           'rsync-system-backup = rsync_system_backup.cli:main',