PluginManager update

Also added Widget.lua. We need to discuss all features of the plugin
soon, and how a cycle goes (check for update, download, install, etc...)

Created account "PluginCentral" on roblox to store our "official"
plugins on. That makes sure people who want to discover plugins fast can
use that default one. Others can create their own collections which
people can load (and set to default).