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.
Metrics/LineLength: Line is too long. [91/80]<br>Style/StringLiterals: Prefer single-quoted strings when you don't need string interpolation or special symbols.