getConfigOption: introduced new method signature with support for a fallback section

This can be used to look for a per-app config directive, but fall back on "system" if not found. Used for logging settings
AgoConnection: added shutdown() method to forcefully terminate the current session (dropping out of any blocking fetch()es)
Various logging improvements
Added AgoApp class, a boilerplate base for all Ago applications
AgoAppified agodrain
Ifdefed a few LOG_xxx constants
Added libboost-thread dependency in debian build control
Wait for gateway to respond to version request
Add datalogger config page (in plugin).
Add multigraph device to display graph on dashboard and mix multiple sensors graph.
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.