Multi-threading for speed

I used an optimized version of the stackblur algorithm, ready for multithreading from [here](http://vitiy.info/stackblur-algorithm-multi-threaded-blur-for-cpp/). I haven't added the alpha preservation back in either JavaBlur or NativeBlur. Are we sure it should do so? I especially like the results of the NativeBlur, (it's different because the C version gets premultipied alpha, whereas the java version does not (but I could change it to do so).

Doing some informal tests on my HTC One (4 cores):
**Java**: From ~400ms to ~130ms
**Native**: From ~230ms to ~50ms (This makes it faster than the renderscript Gaussian blur! Next I'll have to try a renderscript port of the stackblur)