#50: Add convenience Entity.getFor(Class<? extends Component>) method

As mentioned in #50 - "New getComponent overload for Entity". Includes unit test for the new method. Unsure if adding this method pollutes the public API of the library, but since it's so simple to add here's the code anyway.
Could you please add the reason for this? `ComponentMapper` has `O(1)` time for component retrieval while `Entity.getComponent()` has `O(logn)`.