Dynamically get JobContext configuration

As JobContext changed from class to interface between Hadoop 1 and 2,
we get an IncompatibleClassChangeError if we try to use the proxy class
compiled to either of the versions with the runtime of the other.

By getting the configuration method dynamically, we ensure that you can
compile the proxy class with either Hadoop version, and independent of
version used, it should still work together with either runtime.
WIP: non system tests

The current integration test is more of a system test, and while unit
tests aren’t really possible for the hadoop proxies, there needs to be
some cheaper integration tests. As is, the integration tests are
difficult to extend and since they take ages to run, it is not at all
rewarding to do so.