add offsetTop of each of the element's offsetParents
`while(--i)` -> `applyEach` for `changeDivsToEmptyImages`
`while(--i)` -> `applyEach` for `checkImagesNeedReplacing`
Adding dedicated fixtures for responsive images
Using only fixture images, no network call.
Merge pull request #60 from marcinwenger/element-offset-top

add offsetTop of each of the element's offsetParents
Merge branch 'fix-loop-order' of https://github.com/oncletom/Imager.js into oncletom-fix-loop-order
Merge branch 'fix-404' of https://github.com/oncletom/Imager.js into oncletom-fix-404
Extracting the best width value calculation as a reusable static method (`getClosestValue`).

It is going to be reused to compute the best pixel ratio.
Reformatting core tests
Testing `determineAppropriateDevicePixelRatio`
Implementing `determineAppropriateDevicePixelRatio`
Updating the pixel ratio value before each loop, in case the zoom level changed meanwhile.
`availablePixelRatios` documentation.
Merge branch 'feature-availablePixelRatio' of https://github.com/oncletom/Imager.js into oncletom-feature-availablePixelRatio
Changed Ruby version
Sauce Remote Testing
Merge pull request #70 from oncletom/feature-saucelabs

Sauce Remote Testing
Tuning testing configuration

- making sure Firefox is used in `npm run test-watch`
- ensuring the latest version of Sauce connect is used, otherwise it would not connect properly