Default to DATABASE_URL env variable

Dossier now checks for the `DATABASE_URL` environment variable, and if present, merges the configuration with `config/dossier.yml`, otherwise it just uses `config/dossier.yml`

This fixes #33.

If using Heroku, just configure the development and testing environments for dossier. Production(on Heroku) will use `DATABASE_URL` environment variable, so no need to configure.