\Assert\that() fluent API for validation

Using the static API on values is very verbose when checking values against multiple assertions.

This PR adds a much nicer fluent API for assertions, starting with ``\Assert\that($value)`` and then receiving the assertions you want to call on the fluent interface. You only have to specify the `$value` once.

```php
<?php
\Assert\that($value)->notEmpty()->integer();
\Assert\that($value)->nullOr()->string()->startsWith("Foo");
\Assert\that($values)->all()->float();
```