Add constraint factory

[Maks3w]

This factory can be injected and allow extend built-in constraint types with custom ones.

This factory allows extend the constraint system with custom ones.

Example:

<?php

class MyFactory extends JsonSchema\Constraints\Factory {
     public function factory($constraintName) {
           if ($constraintName === 'custom_constraint') {
               return new CustomConstraint($this->uriRetriever);
           }

           return super::factory($constraintName);
     }
}

$validator = new Validator(Constraint::CHECK_MODE_NORMAL, null, new MyFactory());

This especially usefull when you need extend FormatConstraint for to validate custom formats.

[Maks3w]

@bighappyface ping

[bighappyface]

+1

@alecsammon would you mind giving this a review?

[alecsammon]

I think this might be clearer if called getConstraintValidator or something similar. The class is a factory, this is a getter/builder.

[alecsammon]

Hi @Maks3w. Looks like a good PR, is essential that custom validation formats are supported by this library.

I've put quite a few comments on, most of these are to do with my personal style, so happy if you disagree and say no!

Thanks for you patience with this PR.

[Maks3w]

Done

[alecsammon]

Hi @Maks3w - great - thank you...

@bighappyface - one question for you - if the Factory should be renamed to ConstraintFactory

Otherwise I'm happy - once answered then it's ready to merge

+1

[Maks3w]

About class name rules I have two:

1. If there are namespaces there is no need for add suffixes or preffixes except the required by PSR-1
2. If Utility classes will share the namespace and may could be name conflicts inside the namespace then non Utitily classes will be suffixed with the role.

[Maks3w]

Travis-CI error is not related to this changes.

[bighappyface]

@Maks3w would you please rebase this on master for the latest?

[Maks3w]

@bighappyface rebased and squashed

[GrahamCampbell]

Missing |null

[bighappyface]

@Maks3w sorry for the wait on this. I'll try to get this in with #182 as the next release.