Renamed unions to traditional structs to allow for Swift support.

Unfortunately, Swift seems unable to convert `union` types properly.  I converted the `AGKQuad` & `AGKLine` properties to a traditional `struct` types to allow for better inter-compatibility.  If people want to access the `.v` member variable of the `AGKQuad`, they can add this extension to their Swift code:

```Swift
extension AGKQuad {
    var v: [CGPoint] {
    get {
        return [tl, tr, br, bl]
    }
    set {
        tl = newValue[0]
        tr = newValue[1]
        br = newValue[2]
        bl = newValue[3]
    }
    }
}
```

This will unfortunately lose the ability to use the `v` member variable in traditional ObjC code, so perhaps a separate branch devoted to Swift support would be useful.  I'm not sure how often this variable is used outside of the AGGeometryKit framework.