Removed trailing quote from classifiers in setup.py
Add pytest-cov to requirement.txt
Slight format/syntax fixes for setup.py
Add tests for unsuccessful OptimoAPI operations.

- Tests added: test_optimoapi_instantiation, test_unsuccessful_get,
  test_unsuccessful_get_planning_in_progress, test_unsuccessful_stop.

- slight fixes on some typos in the validation error messages.
Rename library to optimo. Refactor project's structure.

- Amend setup.py to reflect the name change.

- Simplify the project structure and make everything available through
  the "optimo" namespace.

- Amend documentation