Added a task_id to better keep track of tasks with the same class.
Fixed a small bug with displaying tasks that are never executed in the
flow graph.
Added an assert to check for scale > 0 in Image.rescale.
A fix for saving opencv machine learning classifiers.
Added better descriptions in the vtm output.
Fixed bad indentation.
Removed some unused tests.
Resoved a confict between subgraph and color additions and the abilty to
add multiple task factories that generate the same tasks.

Merge branch 'master' of https://github.com/bolme/pyvision

Conflicts:
	src/pyvision/beta/vtm.py