Added continuous progress option

Added continuous progress option that allows user to configure Ring color, ring background color and progress update interval. In my app I don't want to be updating the progress indicator so this option keeps the ring progress moving until I call BTProgressHUD.Dismiss(). I also added configurable ring and ring background color as well as the progress update interval.

Example:
```C#
BTProgressHUD.Ring.Color = UIColor.White;
BTProgressHUD.Ring.BackgroundColor = UIColor.DarkGray;
BTProgressHUD.Ring.ProgressUpdateInterval = 300;
BTProgressHUD.ShowContinuousProgress("Continuous progress...");
```
Here are some screenshots from code running on iPod and iPad with a green ring.
![img_0075](https://f.cloud.github.com/assets/1189224/950612/299cbe92-0392-11e3-9dc3-c6c17c63a8f8.PNG)
![img_0300](https://f.cloud.github.com/assets/1189224/950613/2c5108dc-0392-11e3-8156-74151ae55326.PNG)

Let me know what you think of this pull. Thanks Nic