Add trailWidth and centerTail options

Allow users to specify trail width and center tail on the path. 
![screen shot 2014-11-19 at 1 14 35 pm](https://cloud.githubusercontent.com/assets/1610182/5114523/377d01a6-6fee-11e4-86c2-2937837fa258.png)

Sample usage:

            var progressBar = new ProgressBar.Circle(document.getElementById("container"), {
                strokeWidth: 10,
                color: "#fb7d08",
                trailColor: "#9797a5",
                trailWidth: 1,
                centerTrail: true
            });

            progressBar.set(0.3);


            var progressBar2 = new ProgressBar.Circle(document.getElementById("container"), {
                strokeWidth: 10,
                trailWidth: 1,
                trailColor: "#9797a5",
                centerTrail: true,
                color: "#6cb519"
            });