Error collection functions added.

The API now provides functions for collecting errors found during comparing JSON against provided schema. In case an error is met, instance is skipped, while the error is reported to external handler.
Added unit tests for the new error collection feature.

Tests do not cover the core validation functionality, only check the collection feature, i.e. validates the number and format of returned errors for different examples.
Fixed incorrect error format for schema errors
Added new tests for the multiple errors accumulation feature.
Fixed incorrect error format for additional properties error.
Fixed error value for not unique items
Fixed error value for not unique items
Merge pull request #1 from alatnikov/feature_error_list

New feature: return full list of errors on invalid input