Remove our sgp4 subpackage.

I’ve replaced our copies of the JSatTrak code with the JAR file from the official website. This still needs:

* To actually be tested beyond compiling and running!
* Possibly a way to slim down JSatTrak.jar to just the code that we're using (and its dependencies). As is, JSatTrak is ~3 megs.

I'm just putting it on GitHub so that @tommyjasmin has an easy way to get at it.