Implement json path argument in the accumulator callback.

The error accumulator callback takes one new argument which is a json path of the place in the original json where corresponding error happened. Path is always an Erlang string.
Added tests for the error path feature
Fixed union type issue and tests added for this case
Added test case for unique items
Fixed 'dependency' issue (fields mismatch)
Merge pull request #3 from alatnikov/feature_error_path

New feature: json path in the error accumulator callback