GNOME 3.8 support (revised)

Extension works now as expected but I had to disable button in notifications in order to make it work. My guess is that there is a bug in gnome shell (see the stack trace below). Any feedback is appreciated.

Once again sorry for the previous pull request. I should have tested properly.


NOTE:For further information, here is the stack trace when the button is enabled:
`JS ERROR: !!!   Exception was: TypeError: string.substr is not a function
JS ERROR: !!!     message = '"string.substr is not a function"'
JS ERROR: !!!     fileName = '"/usr/share/gnome-shell/js/ui/messageTray.js"'
JS ERROR: !!!     lineNumber = '224'
JS ERROR: !!!     stack = '"strHasSuffix@/usr/share/gnome-shell/js/ui/messageTray.js:224
Notification<.addButton@/usr/share/gnome-shell/js/ui/messageTray.js:698
wrapper@/usr/share/gjs-1.0/lang.js:213
PomodoroTimer<._notifyPomodoroEnd@/home/haaja/.local/share/gnome-shell/extensions/pomodoro@arun.codito.in/timer.js:464
wrapper@/usr/share/gjs-1.0/lang.js:213
PomodoroTimer<._onTimeout@/home/haaja/.local/share/gnome-shell/extensions/pomodoro@arun.codito.in/timer.js:364
wrapper@/usr/share/gjs-1.0/lang.js:213`