Add support for `python -m landslide`

Hello.

This patch adds support for `python -m landslide`. This is helpful in case you have landslide installed with multiple Python versions and you want to access only one of it. On windows, running just `landslide` could give you either the one from Python 2 or the one from Python 3.
Right now, I can use `python -m landslide.main`, but it's nicer to use only `python -m landslide`.
Thank you!

PS: doesn't have tests. :-)