jshint only on build

Run JSHint only in build

See #114. Changes/explanations:

- no need to `require` jshint-stylish ([docs](https://github.com/spenceralger/gulp-jshint#external))
- added [fail reporter](https://github.com/spenceralger/gulp-jshint#fail-reporter) so build halts if there are jshint errors
- changed dependency structure - no need for 'html' task to wait for jshint to be finished
- stopped running jshint on watch

I've also renamed `scripts` => `jshint`, I think this makes more sense now. The name 'scripts' implies parity with 'styles', but these two tasks do very different things (linting vs preprocessing) and have different places in the workflow (especially now jshint isn't part of serve/watch), so the name 'jshint' feels more appropriate. This also leaves the name 'scripts' available for people who want to add script preprocessing (eg coffee), and it would be more obvious for them how to fit their new task into the workflow (as a dep of `html` task, like `styles`).