Slight refactoring on OptimoAPI.

- OptimoAPI no longer returns OptimoResponse objects. It will return
  None(successfull operation), the actual data(on get result) or raise
  an exception if something went wrong.

- Change attributes that imply ranges(coordinates or time related
  attrs) to start_FOO, end_FOO to be more consistent.

- Amend tests.

- Slight fixes on the README.