Debian packaging with pkgr

This is a work in progress, to make changes to the poche code so that it can be easily packaged with [pkgr](http://github.com/crohr/pkgr). The main changes will be related to making the app aware of environment variables used for configuration (namely, `DATABASE_URL` and `POCHE_SALT`).

What still needs to be done:

* remove `install/` folder after installation;
* add a script to populate the database (e.g. `poche run setup`);