add sandbox reset method

`sandbox.reset` allows you to reset all spies in sandbox. see #572 for details
Reviewed the code more carefully - `compact` in fact clears internal fakes collection and surely should not be called in reset. thanks for pointing, I'll update PR.
btw, why `compact` is  so complicated? Is not it enough to call `fakes.length = 0` instead of 
````js
function compact(fakeCollection) {
  var fakes = getFakes(fakeCollection);
  var i = 0;
  while (i < fakes.length) {
    fakes.splice(i, 1);
  }
}
````