Content and menu view controllers can be set by custom segues.

I added `APLSlideMenuContentSegue`,  `APLSlideMenuLeftMenuSegue`, and `APLSlideMenuRightMenuSegue`.

With these segues and storyboard file, no code is required for setting `contentViewController`, ... .

As there is no cocoa API which enumerates the segue names of view controller, each segue must be named `content`, `leftMenu`, and `rightMenu`.

Please update `README.md` file for this new feature. (My English is not so good to write a manual)