Add gradient option to bar & circle charts

Hi, we needed to add gradients to the bar and circle chart. We also needed to refactor the folder structure to work with a private podspec.


Thanks!

![ios simulator screen shot aug 2 2014 10 46 13 pm](https://cloud.githubusercontent.com/assets/601550/3789905/9cf42180-1ad1-11e4-86f7-69538fdcd683.png)
![ios simulator screen shot aug 2 2014 10 46 18 pm](https://cloud.githubusercontent.com/assets/601550/3789906/9cf42c02-1ad1-11e4-92bf-b17efacf6376.png)