Added distro-container.sh script.

This script is a simple wrapper around setup-lang.sh and
polysquare-travis-container, which sets up a fake distribution
container that can be proot'ed into.

Pass a path using -p and be sure to cache it.

Use psq-travis-container-exec (cmd) to execute commands inside
the proot container.
Merge pull request #16 from smspillaz/polysquare-ci-scripts.item_16

Added distro-container.sh script.