Added note re: cloning using -b gh-pages.

1.  Modify README.md and other files to reflect use of GitHub Import.
2.  Modified URLs for shared files in `_include` files to get the right images and CSS.

This turned up #137: the CSS files for the workshop template, the main
website, and the lesson repository are all out of step and need to be
synchronized.  Once that is done, the URLs in the
`_includes/header.html` file should be updated

Add link for "below" installing ruby/github-pages, and disclaimer that these need to be installed, earlier in the document

Address issue reported by @willingc.

We should avoid mixing HTTP and HTTPS. For accomplish this we
are using `//` at the start of the links at the homepage so that
the same protocol is used. For that reason to preview/test the
links you need to launch a HTTP server.

Thanks to @aaren for reviews and comments.

When following the instructions, I tried to run `git push` to push my changes, but the local `gh-pages` branch was still tracking the upstream branch, rather the branch in the `origin` remote repository where I wanted it to go. This adds an explicit example to make sure you try to push to your own repo.

1. Moved the software installation instructions down so that "how I set up" is the first thing readers see.
2. Changed titles.
3. Changes "customization instructions below" to link to `CUSTOMIZATION.md`.
4. Changed nested of software installation instructions so that PyYAML wasn't under Jekyll.

The old instructions would fail on older versions of Git.
No one has complained about the Git commands in the create
script yet, though, so this updates the manual instructions
to more or less follow the exact same steps done by the
create script, including the same Git commands.

Now, instead of creating and cloning an empty repo, users
are instructed to create a new repo, but clone the workshop-template
repo and add their new repo as a remote.

Changed URL in step 4 of the manual installation section so it works

1. Tidied up wording in README.md.
2. Took background and design discussion out of CUSTOMIZATION.md and put it into DESIGN.md.

1. Imported checking script from `swcarpentry/bc` repository.
2. Modified `README.md` to clarify formatting of month names in dates.