Allow configuration of the LABEL property

I wasn't a fan of the default display of notifications because my project directory name wasn't the most descriptive and didn't look great in ALL CAPS.  I realized that this was something that I could solve and that might be helpful to others as well.

`Autotest::Growl::label = "Calculator"`

![Screen Shot](https://f.cloud.github.com/assets/1887797/267837/f3b8a256-8ed3-11e2-9225-f98aa0dde43f.png)