move build and watch to npm script

[campersau]

using https://www.npmjs.com/package/nodemon
supersedes #1364
part of #895

Current console output is quite basic:

less:common
less public/css/styles.css
less:components
less components/app/app.css
less components/commitdiff/commitdiff.css
less components/crash/crash.css
less components/home/home.css
less components/imagediff/imagediff.css
less components/path/path.css
less components/refreshbutton/refreshbutton.css
less components/repository/repository.css
less components/stash/stash.css
less components/textdiff/textdiff.css
less components/branches/branches.css
less components/commit/commit.css
less components/graph/graph.css
less components/header/header.css
less components/staging/staging.css
browserify:common
browserify public/js/ungit.js
browserify:components
browserify components/app/app.bundle.js
browserify components/branches/branches.bundle.js
browserify components/commit/commit.bundle.js
browserify components/crash/crash.bundle.js
browserify components/dialogs/dialogs.bundle.js
browserify components/gitErrors/gitErrors.bundle.js
browserify components/header/header.bundle.js
browserify components/home/home.bundle.js
browserify components/imagediff/imagediff.bundle.js
browserify components/login/login.bundle.js
browserify components/path/path.bundle.js
browserify components/refreshbutton/refreshbutton.bundle.js
browserify components/remotes/remotes.bundle.js
browserify components/repository/repository.bundle.js
browserify components/submodules/submodules.bundle.js
browserify components/stash/stash.bundle.js
browserify components/textdiff/textdiff.bundle.js
browserify components/staging/staging.bundle.js
browserify components/commitdiff/commitdiff.bundle.js
browserify components/graph/graph.bundle.js
copy
copy public/js/raven.min.js
copy public/js/raven.min.js.map

[Hirse]

This setup works, but it feels like a step backwards compared to the grunt setup.

It reruns the entire build on each change instead of just the required steps.
Maybe a better alternative to nodemon would be a combination of concurrently and chokidar.
I'm looking into possible setups.