NOTES.md deleted; obsolete.
Signer mutates message instead of returning a #dup.

This is unfortunate but it paves the way to directly signing
Net::HTTPRequest instances whose #dup method doesn't #dup the internal
headers.

Otherwise, complicated net/http specific code would need to be added to
deep-dup the headers.
Signer uses Net::HTTPRequest not HttpSignatures::Message.
HeaderList
SigningString.
HeaderList and SigningString integrated throughout.
HttpSignatures:: prefixes removed from lib code.
Central up-front loading of all files.

require "http_signatures" loads entire library.  Also, spec_helper
requires http_signatures so individual specs don't need to.
README Usage section updated.