Avoid alpha animation  disabling implicit animations when hiding the pie piece

There is a subtle animation bug that makes the animation a bit clunky (in particular on a real device at 1x speed, but you can observe it also in the simulator).
Please compare the 2 gif that shows the slow-down graph animation. Observe how the alpha of all the pieces (except for the first one) is animated producing a weird effect.
Before the fix
![beforefix](https://cloud.githubusercontent.com/assets/2708453/5427723/0c9d52cc-83aa-11e4-9ec2-019e3efe634a.gif)

After the fix
![afterfix](https://cloud.githubusercontent.com/assets/2708453/5427724/0ca809c4-83aa-11e4-84c7-8d49615704a2.gif)

ps: little trick to slow down animation directly on your simulator or device:
1- Pause the app with the debugger
2- run the following lldb command `p [(CALayer *)[[[[UIApplication sharedApplication] windows] objectAtIndex:0] layer] setSpeed:.1f]`