Running `npm run test-{remote,local}` according to the context.

- moving jshint to `npm run test-pre`
- enabling screen buffer to handle Firefox
- running `npm run test-remote` if there is no PR number
- running `npm run test-local` otherwise
Merge pull request #76 from oncletom/feature-76

Running SauceLabs on branch, locally otherwise
Revealing `availableWidths` bug through tests.
Proper initialisation of Imager instance `availableWidths` option.

- removed `isArray` function
- bootstrapping instance variables
- array/object paths