set segue names from runtime attributes

This allows you to set a value on the Controller called “segueNames” of
type “string” which is a comma separated list of the segue names you
want to put inside the segmented controller.

In other words it replaces the call to `addStoryboardSegments:` with a
property defined on the view controller in the storyboard.

Once the ViewController has been dragged onto the story board you can select it, go to the Identity Inspector and add a User Defined Runtime Attribute called 'segueNames'.
Set its type to "String"
set its value to a comma separated list of the segue names you set in the previous steps, e.g. `first,second,third`.

The SDCSegmentedViewController will internally call `addStoryboardSegments` at runtime with no more code interaction needed.

example screenshot:
![screen shot 2014-08-03 at 16 10 36](https://cloud.githubusercontent.com/assets/316954/3790892/5a683f5a-1b20-11e4-9493-d6485a7a7a43.png)