Add __main__.py

Makes it easier to run (and test) PROPKA as a module.

propka/__main__.py

@@ -0,0 +1,8 @@
+"""Allow the PROPKA package to be run as a program.
+
+For example, `python -m propka`
+"""
+from .run import main
+
+if __name__ == "__main__":
+    main()