First implementation of asserts allowing nulls

[Muusssi]

Mainly an idea for useful extension but not sure this is the best approach to implement these keywords.

Also having trouble with the acceptance tests that seem to fail for master as well and didn't yet have the time to understand what exactly is broken.

[asimell]

@Muusssi I don't see the benefit of writing a completely new keyword with practically identical contents. Instead of having 2 keywords, which call self._type_assertion with True and False you could just add an extra argument, like allow_null and give it a default value of False (as is the current behaviour).