Added temporary vector to adjacent_difference().

Fixes issue #278.
Just FYI, you could also use the iterator-range constructor for [`vector<T>`](http://kylelutz.github.io/compute/boost/compute/vector.html) to do this in one line:

```
vector<value_type> temp(first, last, queue);
```

This will both construct the vector with the proper size and copy the data from the range `[first, last)`.