Periodic termination of canceled/fulfilled instances

A spot request can wind up being fulfilled and hence have an instance
start running at the same time as we have ordered it to be canceled.
This would leave the instance running (costing money), and the adapter
would not terminate it. This should no longer be the case as of this
commit.

Also fixed: a lot of typos where "canceled" was spelled "cancelled",
making it incompatible with Amazon's API.
Merge branch 'master' of https://github.com/elastisys/adapter.spotadapter

Conflicts:
	spotadapter/scalinggroup.py
	tests/test_scalinggroup.py