Redesign auth

Redesigned authentication:

 - implemented a file based credentials/authorization storages;
 - added functions to load credentials/authorizations from either a Go type directly or from a Reader (currently can use a file for this purpose, but nothing stops a DB backend to implement Reader and thus read all the credentials from a DB instead);
 - ensured that passwords are stored using one way hashing function (SHA256 currently);

 Also did a tentative cleanup of the main/command file - moved initialization helpers to their own file.