Classpath-aware schema client, pre-registered schemas & bugfixes

Improvements:

• built-in SchemaClient implementation for using resources on the classpath
• using maven-source-plugin and maven-javadoc-plugin - contributed by @jklukas
• adding SchemaException#toJSON()
• pre-defining schemas by their arbitary URIs before loading via SchemaLoaderBuilder#registerSchemaByURI() (requested in #38 and #212)

Bugfixes:

• correcting schema location of ReferenceSchemas (reported in #233 )
• fixing Schema#toString() calls throwing an exception when the schema has a "title" or "description"
• fixing a bug causing remote-loaded schemas to loose their origins as context "$id"s - reported in #260
• fixing remote schemas not taking into account some loader settings (regexFactory and useDefaults)