README.md: remove SASS reference since it's been removed.
Fix a couple of CSS issues.

* merge duplicate selectors
* remove duplicate properties
* fix `cursor` property in `.sort`
* add `cursor:pointer` in `.displayHelp`
* remove `outline:none` when no other visual change is made
Use a `<p>` for the footer text.
Losslessly compress the base64 png image for search.

It's 1/3 of its original size, for free.