refs #1486 fix website id is not updated but website name if someone switches the website in website selector

refs #1486 added some methods needed in CustomAlerts plugin to validate report and metric is still valid

Refs #4263, display next scheduled run time for geoip updater task, make sure updater is run in next cron run if a file is out of date, & forgot to commit new file in last commit.

Refs #4263, fix bugs in geoip auto updater, show next scheduled time for updater in admin screen and refactor the updater a bit.

Fixes #4483 - thanks for the report that's really useful as loads of users are on cpanel and hostgator!
Also updated the doc at: http://piwik.org/docs/setup-auto-archiving/#cpanel-how-to-set-up-the-cron-script-using-cpanel
and removed http:// in the cpanel examples..