DateTime: createFromFormat() returns Nette\Utils\DateTime instance

Static factory method `createFromFormat()` now returns `Nette\Utils\DateTime` instance instead of PHP's `\DateTime`.

This fixes somewhat unexpected behaviour when calling factory method on a `Nette\Utils\DateTime` class and receiving an instance of different class.

Closes nette/nette#1258.

Note: I used `call_user_func_array()` because the third parameter is optional and cannot be `NULL` for the parent method. I didn't want to use an `if` condition just for this (but if it is found ugly I will update it).