issue_563 Support adding custom message and custom validator

[makeItEasyQ]

Support adding a custom message at attribute level and custom validator for all schema versions. The documented info for the custom validator, only supports one version, but actually, we may use different schemes by different members. So we can provide the JsonMetaSchema#getKeywords.
In this way, we can easy to add additional custom validators. For example:
JsonSchemaVersion jsonSchemaVersion = JsonSchemaFactory.checkVersion(versionFlag); JsonMetaSchema metaSchema = jsonSchemaVersion.getInstance(); metaSchema.getKeywords().put(DATE_PATTERN, new DatePatternKeyWord(DATE_PATTERN)); return JsonSchemaFactory.builder() .defaultMetaSchemaURI(metaSchema.getUri()) .addMetaSchema(metaSchema) .build();

[makeItEasyQ]

Hi @stevehu,
Could you please help review these changes? Thanks very much!

[stevehu]

@makeItEasyQ Thanks a lot for your help. I am in the process of reviewing the code. Look good to me but I need to make sure there is no backward compatibility issue.

[makeItEasyQ]

@stevehu , Thanks very much!