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
[Fix #67] Add `widthInterpolator` option

This allows the widthsMap to be dynamically created using an array of
availableWidths and a function, rather than having to build an
availableWidths object outside of Imager.
Merge pull request #68 from BPScott/width-interpolator

[Fix #67] Add `widthInterpolator` option