Cleaned up constructor reflection code.

There was a lot of duplicate code and tests between BurstAndroid and BurstJUnit4 for reflecting on constructors, refactored all that into BurstableConstructor and BurstableConstructorTest.
Fixed spacing and javadoc. Left the ```@throws``` in there, since I think it's the most descriptive way of saying that multiple constructors aren't allowed. If you don't think that needs to be documented at all, I can remove it, but why wouldn't we want to document that?