Added Pools.set(...)

I've added a small method to `Pools` to be able to set a pool for a class directly instead of having a `ReflectionPool` created automatically. Sometimes a ReflectionPool just isn't possible, for example in case there is no default constructor.

I was also required to change the internal `Map<Class, ReflectionPool>` to a `Map<Class, Pool>` for that reason, which shouldn't be bad though.