Fix for numeric strings as "analytics-value" with GA

Fix for the issue described here:

https://github.com/luisfarzati/angulartics/issues/81

The change is simply to convert properties.value to an int if it's present. I also changed the GA sample, which showed one element with analytics-on but lacking analytics-category, this seems incorrect; in my testing it never fires an event. This sample mislead me into thinking that there was an acceptable default for category.