Add Generics to ViewFrameProperty

[el-hoshino]

Currently ViewFrameProperty's parentView is UIView?. Make it Generics so users can set layout depending on parentView's types. eg., for UIScrollViews subviews can use its contentOffset to setup the layout

[el-hoshino]

Uploaded a branch feature/#62 that contains a part of this implement, but may be impossible to fix this issue, because in Generics type constraints it needs == for type comparing, which will break the code auto-completion, and it's something I don't want.

If Xcode 9.3 with Swift 4.1 solves this code auto-completion issue, I'll keep on implementation