Added Exists Method & Re-activating Stopped Intervals

**Using Version 0.3**
* Added the `exists` method to check if a specific interval is already defined and optionally checks if it is currently stopped
* Updated timer object to include `active` element. This indicates whether the interval is currently stopped
* Updated the `start` method to allow re-activating a previously stopped interval: `Poll.start('name');`
* Updated objects on lines 18 & 19 (now lines 33 & 34) to use dot notation

Here is a fiddle that demonstrates the feature: http://jsfiddle.net/AndrewDavidJ/ZzJ8R/