Fix web UI agodrain

handleEvent signature was changed in ac7eb2ccc
Fix plugin-page links to support relative paths
Fix starting issue (unable to get gateway protocol version)
Add missing ipx image and change img path
Update AgoConnection's command/eventHandler to allow use members of c++ class-members as callback functions

Changes internal impl to boost::function instead of C-style function pointers
AgoConnection: add shutdown signal, to break out of main loop properly if shutdown() is called
Added AgoApp startup error, to abort init
AgoApp: setup default commandHandler and eventHandler in setupAgoConnection
AgoApp: add default appMain impl
AgoApp: Call cleanup on exit
Transform agodatalogger into an AgoApp

This is not a full clean convert, it only moves the strictly required
pieces into a class. This means anything which uses agoConnection which is now
a class member instead of a global var.

A full port should move all global vars and all methods into the class too.
Update pushbullet api uri
Update pushbullet lib to fit to new api version (v2)
Add missing fix for agoalert
Add new required library for agoalert
Fix command handler assignment (we assigned ourselfs..)
Cleaned out old Log class (barely/never used?)
Register existing multigraph when starting
Fix bug when generating thumb graph with only one sensor
Cleanup dup-implementations of getAgocontroller()