Add IBDesignable and IBInspectable

[LeeroyDing]

Supporting them will make life with storyboards easier.

This is a very simple approach without modifying anything. Any discussion is welcome.

[ninjinkun]

I tried this patch through Carthage. But it didn't work because IB annotations were striped from framework interoperability header GradientView-Swift.h. It seems Swift's bug. Same problems are here Carthage/Carthage#335 .
This patch maybe works without framework (add to project directly).

[soffes]

Thanks! Can you remove all of those white space changes from your PR? Guessing those weren't on purpose.

@ninjinkun bummer. I guess people can just use submodules if they want to use this until this is fixed.

[LeeroyDing]

I reversed the whitespace changes but kept one that changed the indention from white spaces to tabs, which stays in sync with other lines. No idea why this line was indented with space.

I wish Apple would make IBInspectable arrays possible, so it would be much more useful.

[soffes]

Looks good! Thanks!