* Add support for react-native 0.52.0

* Refactore a bit

1. Rename `DevCommandsHandlerProxy` to `DevHelperProxy` so it reflects the changes in RN. See d19afc73f5 .
2. Add few comments to show which code is used on `RN >= 0.52` and which on `RN <= 0.51`.

* Now allowing the custom nav bar to take up the whole space on iOS

The custom nav bar on iOS was really just a custom view applied to the title of the navigation bar.
There were problems with this approach, because the navigation bar messes up the frame of the title very frequently, especially when the orientation is changing. That behaviour is accepted when the title is a label, but not when we have a custom component.

With this pull request the custom nav bar component can take up the full width of the navigation bar, no matter what orientation we're at.

* Added mix of native btns and custom component on the Custom TopBar screen of the example project