DropMessage refactoring: Don't use generic-parametered DropMessages

but ModelObjects as `data` field that shall be casted to specific target types afterwards.

Perhaps a reasonable way to get rid of the template-bracket-frenzy that's spread all over the code.
Please review and critizize - I haven't spent so much time in semantically walking through all the changes.

Resolves #199 once merged.
Thanks for the explanation. I also did a bit research. Am I right that this problem is just about being able to get/check the type of a (subclass of) ```ModelObject``` at runtime in a reasonable and comfortable way because we just need to do this in more than one place anyway? If so, I like your approach @Gottox.

I guess you see it more from the core developer point of view and I really needed to point out that this must be nice to use in e.g. modules. We had a -- warning all caps -- CLUSTER FUCK the last time we suddenly needed to use our core system / drop component for building the hello world module example. I want this to not happen again.

Just look at the changes of the multi part crypto test (the lines of code this discussion is based upon). Today you just have to issue
```
DropMessage<TestObject> dropMessage = new DropMessage<TestObject>();
TestObject data = new TestObject();
data.setSomethingSpicific();
dropMessage.setData(data);
```
to create a drop message and ```drop.sendAndForget(dropMessage, contact);``` to send it.
Receiving is even more beautiful:
```
DropMessage<TestObject> msg = mQueue.take();
TestObject data = msg.getData();
data.getSomethingSpecific();
````

I **really** like to keep it that simple. Which must not mean we have to keep it exactly this way but certainly not something like we would get after merging this PR in it's current state (i.e. the lines this discussion is based upon).

tl;dr
the ```as()``` approach is nice to check/get type safety for a (subclass of) a ```ModelObject``` but you should not have to do this manually as a user of qabel core. Of course the core could use it to present a user of the core something casted in a safe way.

Do you guys understand what I try to explain here? Hopefully we are still talking about the same thing :)