Added nullability annotations and nil defaults.

[dpyro]

I added nullability annotations for use in Swift (no more ugly !). I also allowed nil to be used as a default value and in the case of the font names, the default or an unfound font will use the system font (no more crashing in the Interface Builder). Font sizes < 0 revert to the default size, as all values less than 0.0 are not allowed according to the documentation. I had to push some default settings out of the view constructor but I believe this is an acceptable loss: what the default settings are is not important to the user.

[MatiBot]

Sorry it took some time, Could you please fix the conflicts so I can merge your helpful changes? Thanks!

[dpyro]

Looking at the Travis-CI build output, it seems like the compiler is not understanding the “nullable” annotation. The SDK needs to be at least iOS 8.3 for it to work. Is there any way to update Travis-CI to use that?