ThreadService: introduce the getParent(Thread) method

Due to the use of a thread pool, it is impossible to determine which
thread called the current one.

Sometimes we need that, though, e.g. when ImageJ 1.x wants to determine
the (thread-local) macro options and runs in a thread that was spawned
through the thread service.

So let's add API to determine the parent thread, so that the ImageJ 1.x
patcher can be extended to support macro options even when the
ThreadService is involved.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a unit test for the new ThreadService#getParent(Thread) method

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
ThreadServiceTest: add @Before and @After

This is in preparation for additional similar unit tests.
ThreadServiceTest: test more getParent cases
ThreadServiceTest: test the other methods, too
Merge pull request #87 from scijava/get-parent-thread

Introduce ThreadService@getParent(Thread)