Clean up references and add link to Erlay paper and BIP

[michaelfolkson]

Fixes #25

• Added reference to Erlay paper in addition to references to BIP 330 and Rusty mailing list post on possible Lightning application
• Made sure the reference numbers are strictly ascending 1-15 in the reference section
• Updated the numbering in the README so they match up
• Replaced Greg Maxwell links with 404s to web archive links provided by Chris Belcher

Are the reference numbers supposed to be ordered by when they are first referred to in the README or is there some other ordering logic I am unaware of? They generally are but there are a couple of exceptions e.g. reference 14 (previously reference 11) is referred to at the start of the README before say reference 5.