refs #41: phpstan level 3

[mavimo]

refs #41

To merge after #63

[coveralls]

Coverage increased (+0.08%) to 22.97% when pulling 1531590 on feature/41-phpstan-level3 into 6834458 on feature/41-phpstan-level2.

[mickadoo]

it's a pity we have to type hint each API, both inside the method and in the docblock, but if there's no other way to pass this analysis it's not the worst thing in the world

[mavimo]

@mickadoo we can remove docblock annotation when we move to php7 and we type hint methods signature, since we keep support for php 5.6 in this release we should not use type hint in codebase and we need to use annotation.