Add Software.md and some refactorings

* Add _plugins/references.rb to keep all reference links in one place (_references.md)
* Remove index.md (was redundant with about.md)
* Add index.html redirecting to /about/
* Add Software.md
* Rename site title to "Sérgio Miguel Fernandes' homepage"
* Minor _sass/_layout edit to change site's title font size