Make sure InAppBrowser is installed before trying to use Oauth

Added the following to each oauth provider:

```
var cordovaMetadata = cordova.require("cordova/plugin_list").metadata;
```

Per @cfjedimaster it is the best way to see if a plugin was installed:

[http://www.raymondcamden.com/2014/11/19/Determing-installed-plugins-at-runtime-for-Cordova-and-PhoneGap-applications](http://www.raymondcamden.com/2014/11/19/Determing-installed-plugins-at-runtime-for-Cordova-and-PhoneGap-applications)

Throw an error if InAppBrowser is not installed rather than trying to use the baseline **window.open** which will cause unpredictable behavior.