Fix pick function
Fix test
Allow to use klass instead of masks

This allow a neater and more pythonic syntax, e.g:

```python
engine.has_components(state, entity,
                      [RenderComponent, RigidBodyComponent])
```

instead of

```python
engine.has_components(state, entity,
                      RenderComponent.mask | RigidBodyComponent.mask)
```
Remove useless license header

in python/cython source. We already have a copying/license file