登录和修改密码时限制最多输入30个字符， review by zhuxiaojun

@@ -50,6 +50,7 @@ export default class Login extends Component {
                     selectionColor={'white'}
                     autoCapitalize={'none'}
                     autoCorrect={false}
+                    maxLength={30}
                 />
                 <TouchableOpacity
                     onPress={this.props.onDeleteUserPress}>
@@ -74,6 +75,7 @@ export default class Login extends Component {
                     selectionColor={'white'}
                     autoCapitalize={'none'}
                     autoCorrect={false}
+                    maxLength={30}
                 />
                 <TouchableOpacity
                     onPress={this.props.onShowPwdPress}>

@@ -49,7 +49,7 @@ export default class ModifyPassword extends Component {
                   secureTextEntry={true}
                   underlineColorAndroid='transparent'
                   onChangeText={this.props.onModifyOldPwd}
-
+                  maxLength={30}
                 />
               </View>
               <View style={styles.line}/>
@@ -62,7 +62,7 @@ export default class ModifyPassword extends Component {
                   secureTextEntry={true}
                   underlineColorAndroid='transparent'
                   onChangeText={this.props.onModifyNewPwd}
-
+                  maxLength={30}
                 />
               </View>
               <View style={styles.line}/>
@@ -75,7 +75,7 @@ export default class ModifyPassword extends Component {
                   secureTextEntry={true}
                   underlineColorAndroid='transparent'
                   onChangeText={this.props.onModifyRepeatPwd}
-
+                  maxLength={30}
                 />
               </View>
               <Button style={styles.button}