Bug 885233 - Use python2 when available in activation script

The following pull requests fixes the bug [885233](https://bugzilla.mozilla.org/show_bug.cgi?id=885233).

The general procedure is as follows:

If there is a `python2` executable present in the system the activation script will use it and change every occurrence of `python` on lines like `#!/usr/bin/env python`.

This approach has been used for long for the Arch Linux's PKGBUILD which can be found in the AUR (https://aur.archlinux.org/packages/ad/addon-sdk/PKGBUILD)

Please note that running `source bin/activate` takes a bit longer to run for the first time (around 3 seconds on my machine) but then uses the python2 executable without any delay.