Allow placeholders for currying

Allows specifying placeholders for currying. Example:

```php
use React\Curry;

$fn = Curry\bind('substr', Curry\…(), 0, 1);
array_map($fn, array('foo', 'bar', 'baz'));
```

I have to say, that using "…" as function name requires PHP to be compiled with multibyte support. Maybe we need to conditionally define the class and use something less readable like "Curry::placeholder()" for non-multibyte PHP versions.