Add prompt for logout

app/components/navigation/LogoutButton.js

@@ -4,9 +4,10 @@ import Icon from 'react-native-vector-icons/FontAwesome';
 import token from '../../api/token';
 import RouteMaster from '../routes/RouteMaster';
 
-var {
+const {
   StyleSheet,
   TouchableHighlight,
+  Alert,
   View,
   Text
 } = React;
@@ -31,6 +32,16 @@ export default class LogoutButton extends React.Component {
     token.clear();
     this.props.nav.resetTo(RouteMaster.get('LOGIN'));
   }
+  promptLogout() {
+    Alert.alert(
+      'Log out?',
+      'Are you sure you want to log out?',
+      [
+        { text: 'Yes', onPress: this.logout.bind(this) },
+        { text: 'No' }
+      ]
+    )
+  }
 
   render() {
     return (
@@ -38,7 +49,7 @@ export default class LogoutButton extends React.Component {
         <TouchableHighlight
           style={styles.toolbarButton}
           underlayColor={GlobalStyles.get('CIRCLE_NAVBAR_BG')}
-          onPress={this.logout.bind(this)}>
+          onPress={this.promptLogout.bind(this)}>
           <Text style={styles.text}><Icon name="sign-out" style={styles.text}/></Text>
         </TouchableHighlight>
       </View>