Pluggable authentication

This includes the initial concept of pluggable authentication along with the
initial concept for a plugin management system.
Complete initial sketch of plugin (un)install
Make room for single sign on backends

Like HTTP authentication
plugins: Add upgrade script

And add fixup rebase conflicts, and add nice page for unconfigurable plugins
Add Net/DNS2 PEAR library
Merge pull request #236 from greezybacon/feature/plugins

Add plugin concept and plugin management system

Sorta-Reviewed-By: Peter Rotich <peter@osticket.com>