Delete README.md and keep the .rst one
Make various improvements.

- Add new OptimoValidationError that will be raised for higher-level
  model validation errors.

- Move OptimoError and the new OptimoValidationError to errors.py

- SchedulingInfo model now accepts both string driver ids and Driver
  instances for 'schedled_driver' parameter.

- RoutePlan.validate() now ascertains that every mention of a driver id
  inside a SchedulingInfo object corresponds to a driver in
  RoutePlan.drivers

- Remove references of UnavailableTime objects, as they are simply
  TimeWindow objects.

- Amend tests.