Measure functions don't return object like it's now in the binding type.

react-native types : https://github.com/facebook/react-native/blob/master/Libraries/Renderer/shims/ReactNativeTypes.js

I typed callback function directly in the NativeMethods file because it's not a big file, so I think it's easier to find the informations we want about the typing of measure functions for someone browsing source files.

But if you prefer type mesureCallback & co in the NativeTypes file I can update the PR.

Thank's.