Create separate instant cache directory for each python version

[Martin Sandve Alnæs]

Because .instant/cache/ is a python site-packages-kind-of directory, it needs namespacing with the python version for fenics with 2.7 and 3.x to live side by side on the same system.

E.g. by making it .instant/cache/python2.7/ and .instant/cache/python3.4/.

[Prof Garth Wells]

We could fix Issue #5 at the same time.

[Martin Sandve Alnæs]

What we've been doing for some time with the fenics-install scripts in fenics-developer-tools is to create a separate cache directory for each installation profile. That way it's possible to e.g. have a production profile and a developer profile and run instant-clean on the developer profile. Thus in the most general case this is really an installation and system configuration issue.

[Prof Garth Wells]

If this gets fixed, it would be a good time to fix Issue #5 too.

[Martin Sandve Alnæs]

Removing milestone: 1.7 (automated comment)

[Martin Sandve Alnæs]

Made the default ~/.cache/instant/pythonM.N unless in an env, in which case it's not needed.