fixed onLoadResource, added InAppBrowser.addJavaScriptHandler() and InAppBrowser.removeJavaScriptHandler() methods, v0.3.0

v0.2.1, fixed InAppBrowser.injectScriptCode() method when there is not a return value, added InAppBrowser.onConsoleMessage() method to manage console messages #5

added initial support of SFSafariViewController for iOS, updated ChromeCustomTabs for android, added initial version of _ChannelManager dart class