Fixes #746 

These steps are based on my other project React native draftjs which requires loading a local HTML file. The code can be found in the following line - bc51410117/index.js (L113)

[skip ci]

* Added guide for working with custom headers and cookies

* Remove extraneous files

Apple required us to remove this (see #819)

BREAKING CHANGE: UIWebView has been removed
BREAKING CHANGE: useWebkit prop removal
BREAKING CHANGE: scalesPageToFit prop removal on iOS (since it's not compatible with WKWebview)
BREAKING CHANGE: Renamed RNCWKWebView to RNCWebView on iOS

* Added guides for communication between React Native and web pages

* Updated Guide a bit

* Adds guide for handling navigation state changes

* Update Guide.md

Addresses #80.

Caveat: I am not an Android developer. This code comes from a fork of the original RN WebView that we have been using in production for some time, so all credit goes to @Oblongmana: https://github.com/Oblongmana/react-native-webview-file-upload-android.

Setting up a DownloadManager for the WebView is pretty straightforward, as is adding any known cookies to the request. Most of the complication comes from the requirement after SDK 23 to ask the user for the WRITE_EXTERNAL_STORAGE permission. Unfortunately there is no mechanism to suspend the download request until permission is resolved so this code stores off the request and sets up a listener that enqueues the download once permissions are resolved so the user experience is really nice.

I didn't see anything in the way of tests or documentation that needs to be added for this change, so let me know if I missed anything. Thanks!

Fixes #33 

I could really use some help from an Android developer on this one, because I just "made it work", don't know how to "make it work good".

Some things that should be reviewed:

- [ ] validate Android 5.0 devices (my emulator work, but outputs some weird sounds; a Galaxy 4 I tested on crashes)
- [ ] validate Android 5.1 devices (emulator works, couldn't find a real device)
- [ ] how to handle File Extensions? (https://www.w3schools.com/tags/att_input_accept.asp)

I'm sure that there's more refactoring to be done, so any help and advice would be appreciated.