Time bound solvers

[wrigjl]

This is an implementation of time-bound solvers using cooperative threading. There's no functional change for most of the solvers, but the idea is that each solver can check to see if it has run out of time to find a solution. If so, it should gracefully return. The main loop of a solver using this framework would look like:

for (int i = 0; i < Math.pow(2,N); i++) {
   try_one_combination();
  if (timerHasExpired)
    return "timeout";
}

Note: it's not possible in modern threading systems, C# included, to abort a thread AND have a consistent system (what if it was holding a lock? a semaphore? etc.). So, the best you can do is cooperative threading where you ask politely for a thread to finish what it's doing and then wait.

It's annoying that interfaces can't define instance variables, which is the only reason I had to modify every solver. I considered using a class, but that was even uglier.

TODO:

• don't hard code the solution time to 5 seconds (maybe a variable in the json config?)
• go through the solvers and add timer expired checks (can be done after this is merged)

closes #56