Make native blur faster

Use multithreading and a faster implemenation of the stackblur
algorithm.
Make JavaBlur faster

Use multithreading for the java version also
Don't blur the alpha channel

Note that in the native version we clamp the rgb values from 0-alpha,
because the native version uses premultiplied alpha.
Merge branch 'master' into fast_blur
Port the stackblur algorithm to renderscript

Stop using the Intrinsic Gaussian Blur, and move to a port of the
stackblur algorithm. This should be a good deal faster.
Merge pull request #11 from Dr-Emann/fast_blur

Multi-threading for speed