Bug 933192 - [Settings] can join hidden wifi network without ssid name r=ejchen

a)target:prevent from joining hidden wifi network if any of the  fields are not valid:1)the ssid name2)the user name 3)the password
b)scheme:
three js files are modified during the process:shared/js/wifi_helper.js, apps/ftu/js/ui.js, apps/settings/js/wifi.js,
1)in shared/js/wifi_helper.js:
Parameter ssid is added in function isValidInput
2)apps/ftu/js/ui.js:
beside 'password' text,we also add event listener  to 'ssid' and 'user name' text boxes .the above three listeners could listen for any changes happened in the three text boxes and then disable or enable the join button
3)apps/settings/js/wifi.js:
beside 'password' text,we also add event listener  to 'ssid' text box .the above two listeners could listen for any changes happened in the two text boxes and then disable or enable the ok button