Convert to markdown

[markburns]

As per GitHub's email, they are no longer supporting textile.

This PR converts to markdown, whilst ensuring that file renames are tracked by git.

I've also done a wget on each of the pages straight from http://ruby-hacking-guide.github.io/ and saved in the html_current directory.

I'm not sure if Github Pages will build pull requests or not.

I propose this:

• I wait a few days to see if anyone objects.
• Assuming not, merge this in.
• wget the new pages and commit.
• Open issues to fix any issues if minor OR revert the PR if it's a disaster