Make ICortex work on Google Colab, doc improvements

[osolmaz]

• Google Colab doesn't allow installing new kernels, so I shifted the internals around a bit and made it possible to overload an existing IPython shell with ICortex magics by just running:

import icortex.init

• Improved README and quickstart page to be less obscure.
• Lowered version dependencies to Python 3.7 because Google Colab was installing an older version of ICortex. Not sure whether it's a caching problem or a versioning problem, will make a release to see whether this resolves it.

[osolmaz]

It worked, #28 is resolved.