Document the various plugins

[sushimustwrite]

See photo/frontend#1288 for the original issue.

[jmathai]

This should list the available plugins and what they do as well as document how others can write their own plugins. See photo/frontend#1288

[mixedbredie]

These are the plugins available currently in 4.0.1 and all are deactivated by default except for the tutorial plugin. They can all be activated. Clicking the configure link opens a panel asking me to upgrade the plugin then save. Clicking save returns an error.

BetterPageTitles
EmailNotification
FacebookConnect
FacebookLike
Gauges
GoogleAnalytics
HelloWorld
Piwik
Tutorial
TwitterCard