Algorithm Executor Service

This branch is simply a cleaned-up version of:

https://github.com/imglib/imglib/tree/gauss-executionservice

I took the current master and reimplemented everything from gauss-executorservice. 

@tpietzsch: Could you review it? You can also delete gauss-executor-service afterwards.
@StephanPreibisch: Do you think you could adapt FFTConv (as discussed here: https://github.com/imglib/imglib/pull/56?)