Fix use of `'required': false` in draft 3 strict mode
Merge pull request #134 from seomoz/fix-draft3-strict-mode

Add support for `"required": false` while using draft3 with strict mode
Adding Ben Kirzhner to contributors
Extract format validation into custom classes
Define available formats in drafts
Adjust Draft4 tests according to predefined format keywords
Ensure correct order of requires
First prototype for custom format validation
load custom format error in custom format attribute file
Test type conformity in format base class
Improve error message for custom formats
Refactor custom format registration
Move data validity check into attribute class
Enable deregistration of custom format validators
Remove superfluous case statement
Allow overriding default formats and add tests
Make proc syntax 1.9.3 compliant
Fix wrong file names