Better exceptions when schema loading fails

Improvements

• better exceptions when schema loading fails - addresses #24
• also collecting all failures rather than failing on the first (example of programmatic error consumption is here)

Custom format support

Improvements

• enabling users to define their custom "format" validators, as shown in this example - requested by @alturkovic

Bugfixes

• fixing keyword in UnevaluatedPropertiesValidationFailure - reported by @danielaparker

$dynamicRef and time format improvements

Improvements

• improving "$dynamicRef" implementation so that it also searches for potentially "$dynamicAnchor"-ed schemas under "$defs"
• fixing "format": "time" validation, to pass all tests from the testsuite

API compatibility fix

Bugfixes

• fixing API-compatibility of ValidatorConfig class so that new ValidatorConfig(FormatValidationPolicy.ALWAYS) compiles again (it was broken in 0.12.0)

Improvements

• junit and assertj dependency upgrades

Read-write context

Improvements

• adding support for validation of "readOnly" and "writeOnly" keywords, depending on context - requested by @mszawurski in #30
• allowing pre-registering schemas with relative URIs - contributed by @rayokota

Bugfixes

• improving error message for invalid schemas - contributed by @rayokota

Improved format validation policy and pre-registering schemas by URI

Improvements

• added FormatValidationPolicy to make enabling of format validation configurable
• added ability to pre-register schemas by URIs which can be used as references

Fixed issue with subschema internal references

Bug fixes

• #16

Unprocessed props & better dynamic refs

Improvements

• Supporting unprocessed schema properties - contributed by @rayokota
• Improved support for dynamic references

Bugfixes

• fixed an error occuring when a validation failure happens at the same instance nesting level, but on different subschemas

Android compatibility fix

Fixes #18

Loading schemas from URLs

Improvements

• added support to load schemas from URLs with SchemaLoader.forURL("...")
• built-in support for classpath:// protocol included