[Validator] Added comparison validators.
merged branch danielholmes/comparison_validators (PR #790)

This PR was merged into the master branch.

Discussion
----------

[2.3] [Validator] added comparison constraints and validators

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

They work on a class level and you specify a list of properties. Included are the standard GreaterThan, GreaterThanOrEqual, LessThan, LessThanOrEqual and Equal. e.g.:

```php
<?php
/**
 * @assert:GreaterThan({"diedDate", "bornDate"})
 * @assert:LessThanOrEqual({"bornDate", "firstSchoolDayDate", "diedDate"})
 */
class Person
{
    private $bornDate;
    private $firstSchoolDayDate;
    private $diedDate;
}
```

I think it would also be useful if they worked on a property level rather than just class. I'm not sure what the default option should be though. e.g. is there a reliable way to determine what's a raw value and what's a property name:

```php
<?php
/** @assert:GreaterThan(80) */
private $iq;
/** @assert:LessThan('dateDied') */
private $bornDate;
/** @assert:LessThanOrEqual('C') */
private $grade;
/** @assert:GreaterThanOrEqual({50, 'ageStartedSchool'}) */
private $age;
```

Commits
-------

0bffdff [Validator] Added comparison validators.