[Form][Validator] Extended Date Validator with before/after options

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #3640
| License       | MIT

Due to personal use I extended the DateValidator to handle `after` and `before` options. The existing validation of the value being an actual date is still there.

Example:

    // only dates within the year 2014 will be valid.
    new Assert\Date(array( 
        'after'   => '2013-12-31'
        'before'  => '2015-01-01'
    )),

It is possible to change the date format in error messages.
In the example below, an error message for the value '2016-01-01' would look like `The date should be before 01.01.2015`

    new Assert\Date(array( 
        'after'   => '2013-12-31'
        'before'  => '2015-01-01'
        'message' => 'This value is not a valid date',
        'messageAfterDate'  => 'The date should be after {{ after }}',
        'messageBeforeDate' => 'The date should be before {{ before }}',
        'dateFormatMessages' => 'dd.MM.yyyy'
    )),

If needed, the DateFormatter can be exchanged completely:

    new Assert\Date(array( 
        'after'   => '2013-12-31'
        'messageAfterDate'  => 'The date should be after {{ after }}',
        'dateFormatter' =>  new \IntlDateFormatter('de_DE', \IntlDateFormatter::MEDIUM, \IntlDateFormatter::NONE)
    )),