Added elastic boolean to let the user pan the center view controller past the edge

[chourobin]

Wanted to add an elastic effect similar to the Wunderlist 2 iOS app. To keep consistent with existing API, elastic is off by default and needs to be enabled.

Let me know what you think?

[mikefrederick]

I dig it. I would even make this the default behavior (possibly even remove the option).

However, right now the library resizes the side menu to fit to inside the leftMenuWidth/rightMenuWidth properties, which makes this behavior odd. There is a black space behind the centerViewController when it is pulled past the menu.

Maybe it is better to not adjust the frame of the menus and leave that work up to people using the library.