Possibility to use RenderScript

[amasciul]

This PR adds the possibility of using RenderScript for blurring. Sample has been updated with a checkbox to enable RenderScript.

• By default, the library does not use RenderScript.
• API is backward compatible with identical behaviour.
• I chose not to use RenderScript retrocompatibilty to API level >= 8, as it forces the developer of the final app to modify his build.gradle file (see http://android-developers.blogspot.fr/2013/09/renderscript-in-android-support-library.html), but it's up to you to change that if you want ;)

[tbarthel-fr]

That could definitly be very useful (:

RenderScript will be available in the next release!

Thanks a lot :D