Build: Export build_branch for gh-pages variable
Merge pull request #3149 from nschonni/v4-export-build_branch

Build: Export build_branch for gh-pages variable
Build: changes to `watch` task

Watch task now invokes `dist` when change detected.
Test: added Sinon test library

Also put devDependencies in alphabetical order because I'm OCD.
Test: added data-picture unit tests