Update Imager.js

Add support for alt attribute using data-alt
Add handling data-alt
add offsetTop of each of the element's offsetParents
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`
Merge pull request #60 from marcinwenger/element-offset-top

add offsetTop of each of the element's offsetParents