Webview settings intercept

[airandfingers]

Messily fixes #57 for Salesforce authentication by adding a "webviewSettingsIntercept" callback option to ITnsOAuthOptionsSalesforce

[airandfingers]

I know these changes have a few problems:

1. They only apply to the Salesforce auth method, because that's all I'm using.

2. webviewSettingsIntercept is treated as a required argument.

3. The callback is called 50+ times per login, when using our Salesforce auth servers.

For 1 and 2, I can add this option to other auth methods and make it optional, if that's the route @alexziskind1 wants to take.

As for 3, I'm not sure where else to call the callback from; I tried calling it from TnsOAuthPageProvider:loginPageFunc, but webview.android and webview.ios were undefined.

Also, it may be better to expose the webview as a property instead of having API consumers pass a callback. However, I'm not sure how this would work - perhaps adding getWebview functions to multiple layers, instead of passing webviewSettingsIntercept during initialization.