ElggVersion object and widget in admin panel

We have here in fact 3 elements:

* ElggVersion object with ability to check latest version of elgg
* version widget in admin panel
* ability to refresh particular widget with link inside of it's body (used in version widget for Update button)

Latest version is determined from:
* first source is most recent tag on github
* if first one fails, we extract version from elgg.org mainpage (dedicated API would be nicer though)
* result is stored for 24h  before automatic refresh if not forced to be updated