Racker exports ENV vars from .env file

It will export any environment variable defined in the `.env` file
before starting your application, hence making them available
without requiring the `dotenv` gem (for example).

It will also source any `.praxrc` file so you may configure things
to your needs (eg: chruby or something). This must be a bash script.
Fix: restart and autokill respawned apps after TTL

Two bugs were fixed: the first one prevented apps to be restarted
using `prax restart` (or `touch tmp/restart.txt`); the second one
prevented Prax::Monitor to autokill an app once it had kill it once.