Fix iOS plugin SetMargins

* Fix SetMargins for Landscape mode.
* Additional SetFrame function so the window can be set from a center point and size.
iOS: add SetFrame function
webview object: add SetCenterPositionWithScale for iOS and Mac
Merge pull request #13 from endavid/fix/iOSLandscape

Fix iOS plugin SetMargins
Check if the keyboard is active from the Webview plugin

* Unity's TouchScreenKeyboard.visible always returns false on Android when the keyboard is active from the plugin.
* With this fix, you'll be able to know when the soft keyboard has been opened in Android.
* For other platforms, it will just default to Unity's TouchScreenKeyboard.visible
Merge pull request #14 from endavid/feature/isKeyboardVisible

Check if the keyboard is active from the Webview plugin
Update README.md
Merge commit '6f64e9db339305f892082a69a1cb200ae594d30d'

Conflicts:
	plugins/WebViewObject.cs