Race condition in Scanner ctor

[mmimica]

There is a race condition in Scanner constructor when counting the number of active Scanner instances.
The increment and decrement of counter in the constructor and close() are not atomic, leading to the wrong value being counted.

The variable should be AtomicInteger.

    public Scanner() {
        // <snip>
        count++;
    }

    @Override
    public void close() {
        scratch.close();
        matchHandler.close();
        count--;
        scratch = null;
    }

[mmimica]

#159