Rewrite readme

This PR fully rewrites the Readme page which is used as the landing page of the Github repository as well as the landing page of the Pypi page https://pypi.python.org/pypi/scc.

These improvement should improve the description of the requirements for installing and testing the package. More importantly, it should clarify the minimal setup required for running the integration test suite of scc.

/cc @hflynn, @mtbc