Make use of GrabHelper in notification dialog

GrabHelper (available since gnome-shell 3.6) has advanage of being able to handle a case when the grab is lost for some reason.
Reduce number of reminders to one
Added support for GNOME 3.8

Fixes #92
Merge pull request #93 from haaja/master

Support for GNOME 3.8
Support for GNOME 3.8 (see note)

NOTE: Extension works now as expected but a button in notifications had to be
disabled due to a bug in gnome-shell.

Stack trace:
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
Merge pull request #94 from haaja/master

GNOME 3.8 support (revised)
Simple clean up

Remove unnecessary const and commented function call.
Update metadata.json, compatibility with gnome-shell 3.6 has been broken
Merge pull request #95 from haaja/master

Remove unnecessary const
Base X11 event capture on IdleMonitor

Conflicts:
	src/timer.js
pt_BR translation included.
Enable Brazilian Portuguese translation
Migration from event-capture to IdleMonitor fixes

and account gnome-shell 3.8 API changes in IdleMonitor.
Migrate toggle-timer keybinding to gnome-shell 3.8
Fix not applying full reset when the timer is stopped
Release version 0.8
Insert info from Configuration wiki to README.md
Update es.po

Fixed a small error, added translations
Merge pull request #97 from UnsolvedCypher/patch-1

Update es.po
Update es.po

Sorry for a second pull request so soon, but I fixed some more errors and added another translation.