Add support for wacom tablet pen pressure

![pressure](https://f.cloud.github.com/assets/690661/820677/45ffd7b2-efca-11e2-8286-8f7746d6916c.png)

I think your tool is really great! It's feel like z-brush in the web!!
I applied tablet pen pressure to brush radius.
Sorry for my little english that I can't explain exactly about it..

wacom web plugin document: http://www.wacomeng.com/web/