Update plugin API.
Attempt to use python-amqp.

python-librabbitmq is broken, see
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=736348

Closes #243.
Fix bad imports.
Update plugin API.
Undo change to GRAPH_ROOT.

Not ideal, but needed until we can fix Apache config mess.
Use FILES_DIR and FILES_URL for graphs.