Add SfxrEditor

As discussed, I added the SfxrEditor class as an initial implementation of a Unity Editor for SfxrParams values  (screenshots included).

![usfxr-menu](https://f.cloud.github.com/assets/2177108/537799/e565cb7c-c17b-11e2-91f8-d7e2c7cf2a21.png)
![usfxr-editor](https://f.cloud.github.com/assets/2177108/537800/e5846fd2-c17b-11e2-8002-e6b9820f01d1.png)