Rework setup/launch process

[abretaud]

Here's a PR with several changes, I prefer to let you have a look at it before merging as I touched many things

• New minor drupal version
• Pre-download tripal code and default modules (fixes #18)
• As it is pre-downloaded it will allow us to pin tripal/module versions => we can make a stable 2.1 image
• Reworked the installation of modules/themes, it should be more robust: no risk of unexpected module updates when restarting the docker, works better with mounted volumes
• Now creating the db if it does not exist (fixes #19, but we can still merge #20 too to use postgres as the default db name)

ping @spficklin too

[hexylena]

This looks amazing @abretaud. Everything is split apart very cleanly, so much better than it was. Excellent!!