Object managers extracted to separate classes

I'm not sure if this approach is better than the one we currently use, but I like it better.

- Introduced object managers
- Methods "smth_do_foo" replaced with "smth.do_foo" syntax
- Sync API default URL upgraded from 5.3 to 5.5
- Added some extra methods in Model classes
- Improved Model.__repr__, defined API.__repr__ methods for the sake of
  convenience