fix Imagick::count() with PHP 5.6.0RC2

With PHP 5.6.0RC2, tests/021-countable.phpt fails:

    TEST 21/27 [tests/021-countable.phpt]
    ========DIFF========
    001+ Warning: Imagick::count() expects exactly 0 parameters, 1 given in /dev/shm/BUILD/php-pecl-imagick-3.2.0/NTS/tests/021-countable.php on line 10
    002+ 0
    001- 3
    ========DONE========
    FAIL Test countable interface [tests/021-countable.phpt] 

Implementing the count_elements handler solves this issue.
Thanks for the PR - but this is the wrong way round isn't it? Looking at the [array handling code](http://lxr.php.net/xref/PHP_5_6/ext/standard/array.c#327) if HAVE_SPL is defined, the code will go through the existing 'count' function.

It's only if HAVE_SPL is not defined that the php_imagick_count_elements would need to be bound to `imagick_object_handlers.count_elements` right? Or am I misreading it?

btw the [extra patch](https://bugs.php.net/patch-display.php?bug=67064&patch=bug67064-BC&revision=1402667581) to revert the BC break to calling count is probably going to be applied, but I will also correct the method in here as it should be following the interface definition.