Bug 881957 - Implement toast and show toast after successfully change PIN r=crh0716

1. Implement toaster under shared folder, basically following design of [building block](https://developer.mozilla.org/en-US/Apps/Design/Building_Blocks)
2. Unit test of toaster 
3. Display toast when user successfully change PIN


[Link to Bug 881957](https://bugzilla.mozilla.org/show_bug.cgi?id=881957)