extend added

Hi
I need long time lock. So, I added extend function.
Below code uses this feature ->
```javascript
var lock = require('redis-lock')(redis_client)

function long_lock (next) {
  lock("working", 10000, function(done, extend) {
    var end, tid, _loop;
    tid = null;
    (_loop = function() {
      return extend(function(err) {
        if (err) {
          console.log("lock error : " + err);
          tid = null;
          return;
        }
        return tid = setTimeout(_loop, 5000);
      });
    })();
    end = function() {
      if (tid != null) {
        clearTimeout(tid);
      }
      return done();
    };
    return next(end);
  });
};

long_lock(function (unlock) {
  doSomeThing(function (){
    unlock();
  });
});
```