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
Format error messages uniformly.

Every error message now contains a piece of JSON instance and a corresponding schema term with the brief description of an error in form of tuple or single atom.
README.md modified in accordance with the new error format
Merge pull request #2 from alatnikov/feature_error_format

New feature: Format error messages uniformly
Added new third-party tool: 'jesse'
Merge branch 'master' of github.com:alertlogic/jesse
Rebase with the latest origin