Added maskOffset property to extent the mask for large font sizes

I have an FXLabel that the user can pinch to zoom and at large font sizes the mask is clipped at the top.

Ive added a maskOffset so that it can be extended when needed

![screen shot 2013-08-30 at 10 29 41 pm](https://f.cloud.github.com/assets/4196665/1056875/9a0b49ae-1162-11e3-8c7b-b1173a9e6773.png)

This is a great class btw!