Refactor time bound associations

Trying to minimze tech debt, so this changes association declarations on various models with aim to simplify them. Check the   changes or summary here https://gist.github.com/1b4ac759cfd68563f0e9. It appears that we don't need custom eager loaders and just have to properly utilize time machine scope on association dataset. As a consequence this change improves render time for goods page as now eager loading works better (sequel must know better how to eager load).

In addition to that I trimmed some unnecessary methods from TimeMachine and added some documentation.

Also added description methods on TariffValidationMatchers in rspec so that they won't throw warning when run with documentation formatter.