[Runtime] Add "-c" option for xwalkctl for restarting aborted install/update tasks

Crosswalk runtime should have a mechanism to restart the aborted install or
update tasks which are stopped by accident like power off. This patch will
enable this feature, user can restart these tasks by command 'xwalkctl
-c|--continue'. This patch related to bug:
https://crosswalk-project.org/jira/browse/XWALK-1565
Okay, I'll remove the return value, ```Install()``` and ```Update()``` return value do not related to whether the tasks have been finished.