Rewrite resource validator and add some tests

New resource validator doesn't suffer the problem of returning error for
has_many association object fields
Add more tests for resource validator
Use activerecord.errors.messages.[model_name] as scope for error localizations for ResourceValidator
Merge branch 'master' into validator_fix

Conflicts:
	CHANGELOG.md
Properly handle errors on base
Fix resource_validator (for content controller)
Move some files from releaf-core/lib to releaf-core/app/lib

That way, these files will be auto reloaded if they change (helps
development)
Rename Releaf::ResourceValidator to Releaf::ErrorFormatter

rename .build_validation_errors to .format_errors