cli: capture() allows only 1 connected device

Relax the test for the number of connected devices from 2 to
either 1 or two.
Workflow: support single camera operation

Add a new property to Workflow objects to return True when a
single camera is in play, and use it to increment the page number
propertly in _get_next_capture_page().
gui: update the capture page for single camera operation

When only a single camera is in use, display a single image
on each capture instead of two.
Fix PEP8 complaints from Travis CI
cli: add missing whitespace in message about whitespace :)
Merge branch 'master' into single_camera
Merge branch 'single_camera' of https://github.com/gareth8118/spreads into gareth8118-single_camera

Conflicts:
	spreadsplug/gui/gui.py
Merge branch 'gareth8118-single_camera'