Adjustments to versioning display

I don't always have a release tag I want to show, so Lanyon's `site.version`
with the prefixed v didn't work for me. This commit adjusts it so you can name
a literal branch (eg `master`) or a tag, and still have a sensible download
link.
Exclude `README.md` and `LICENSE.md`

These files have no YAML front matter, so Jekyll does not parse them. However,
it does include them in the generated site under their original names. Try
http://lanyon.getpoole.com/README.md and you'll see what happens. I've
excluded them so they don't get built in; it would be better to use `about.md`
for a similar purpose instead.
Shorted .gitignore

I prefer to version my drafts separately so that they don't get pushed to
GitHub, hence the ignore entry for `_drafts`.