[Validator] DateRange constraint added

Added DateRange constraint and appropriate translations in russian and english. 

#### Simple example:
```php
    /**
     * @Assert\DateRange(
     *      min    = "01/01/1930",
     *      max    = "01/01/2012",
     * )
     */
    protected $birthday;
```
#### Custom format example:
Date format by default is d/m/Y, but easily can be overridden by 'format' property of constraint, f.e.:
```php
    /**
     * @Assert\DateRange(
     *      min    = "01.01.1930",
     *      max    = "01.01.2012",
     *      format = "d.m.Y"
     * )
     */
    protected $birthday;
```
This is my first commit to symfony components so feel free to request any additionals - write docs, improve something and so on.