Use Atom's notification-manger and keep update history

[wufuC]

Added functionalities:

• Notify user about successful updates using Atom's notification-manager (Fix #4, #14, and #24)
• Keep an update history at ~/.atom/storage/auto-update-packages-update-history (Fix #3 ??)

ps. Codes for file IO are moved to a separate module and imported/required when the last-update-time check is needed. It seems to me that this slightly reduce the package loading time when a new window is drawn.

[rugk]

Any news?

[equinox]

@rugk It's much easier to just go to https://github.com/wufuC/atom-autoupdate-packages (if you haven't already)