Validation failure reporting improvements

Improvements

• added support to inspect which keyword was violated - kudos @rodrigosaito for implementing this
• exposing raw programmer-readable error message (without the pointer to violation) in ValidationException#getErrorMessage() - thanks for @fernandostoiano
• integration testing improvements: added mechanism for specifying expected exceptions. Credits go for @twbutler , docs here
• added method for checking if a given property is defined by a schema instance: Schema#definesProperty() - thanks for @v1ctor for implementing this feature
• combined schemas (allOf, anyOf , oneOf): reporting the exceptions of sub-schemas as failure causes (requested in #33)
• improved way of counting causing exceptions (requested in #36)
• validation failures now can be reported in JSON using ValidationException#toJSON(), docs here
• upgrading to org.json:json:20160810

Bugfixes

• fetching $ref references while loading schema objects even if "type" is also specified (reported in #38 )
• bugfix in "enum" handling: now object comparison doesn't depend on the order of property names returned by JSONObject#getNames()