Catching all errors

[gsakkis]

This PR addresses #11 by making possible to catch all validation errors of a given input instead of just the first one encountered. Any feedback from valideer users that would find this enhancement useful would be most welcome before merging it.

Summary:

• A new Validator.full_validate() method is introduced, similar to validate(). The difference is that invalid inputs raise a MultipleValidationError exception, a new ValidationError subclass with an errors attribute that captures all individual errors in a (flat) list.
• A new ContainerValidator abstract helper class is introduced to act as a base class of container-like validators that need to report multiple errors, without duplicating the logic between validate and full_validate or making the former less efficient than necessary by delegating to the latter.
• All included container validators, namely HomogeneousSequence, HeterogeneousSequence, Mapping and Object now extend ContainerValidator and support reporting mutiple errors out of the box.
• The custom compat.py module for Python 2/3 compatibility is replaced by six.
• All error related classes and functions are moved to a separate errors.py module.

[coveralls]

Coverage increased (+0.21%) to 100.0% when pulling ae44ee1 on catch_all_errors into 133618c on master.