Avoid protocol relative links

I prefixed the protocol relative links (using `https:` where applicable) since they were not being recognized by GitHub Flavored Markdown - see [issue 98](https://github.com/github/github-flavored-markdown/issues/98). Added a backslash to escape the closing parenthesis to the link to "Shim_(computing)" at Wikipedia. 
Merge pull request #89 from davidl/patch-1

Avoid protocol relative links in readme.md