Improved RegEx support

Improvements:

• configurable RegExp implementations (java.util.regex or RE2J) (regression reported in #156 )

Bugfixes:

• fixing bug related to "format" handling in referred schemas (reported in #160)
• properly handling "$schema" values in root documents of referred schemas (reported in #164)
• fixing exception thrown for unrecognized "$schema" values (reported in #170)
• fixing missing SchemaException#schemaLocation property when an unrecognized "type" value is found (reported in #168)
• fixing "$ref" resolution for json pointers which' document-relative part is the same but they point to different schema documents
• fixing a long-standing problem of keywords belonging to separate schema classes being used in the same schema json object (reported in #184 , #174 , #116 , #87 and maybe more)