Update Imager.js

Add support for alt attribute using data-alt
Add handling data-alt
add offsetTop of each of the element's offsetParents
`while(--i)` -> `applyEach` for `changeDivsToEmptyImages`
`while(--i)` -> `applyEach` for `checkImagesNeedReplacing`
Merge pull request #57 from longprao/patch-1

Update Imager.js: Add support for alt attribute using data-alt
Add documentation of data-alt attribute
Add data-alt and alt attributes in sample code
Moving `data-alt` example to the relevant README location
Moving `data-*` tests in a separate file
Testing the 2 use cases of `data-alt`
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`