Simple token authentication

This is implementation for #48. One thing is missing, that is used tokens tracking – I've created `last_used_at` column in table, but I didn't implement it as it is not important, and it should be probably discussed first.
Style/BracesAroundHashParameters: Redundant curly braces around a hash parameter.<br>Style/SpaceInsideHashLiteralBraces: Space inside { missing.<br>Style/SpaceInsideHashLiteralBraces: Space inside } missing.<br>Style/SpaceInsideParens: Space inside parentheses detected.