Hoist commands/helpers into a per-command structure

This is preparation for introducing new binaries
Hoist the specs into a per-binary spec structure
Rename ui_spec to command_spec
Extract command/helper spec assistance into spec_helper
Pull out two helpers into the shared level

Enhance GitHub.load to load both per-bin and shared if both exist
Tweak Rakefile spec tasks to be a bit cleaner
Add gist binary

Implement home command and homepage/homepage_for helper methods