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
Style fixes
Update docs
Add license to spreadsplug.__init__
chdkcamera: Fix old prepare_capture call
gphoto2: Pseudo-implement update_configuration
gui: Fix beeping on error log
web: Refactoring and HTTP API documentation
Ensure all modules have unicode_literals imported
docs: Add missing files
Check PEP8 naming conventions
doc: Reinstate requirements.txt for RTD
docs: Add httpdomain to requirements
doc: Add aafig extension to requirements
doc: Remove aafig dependency