Entity.SignIdentity used the wrong signature function and produced invalid
certification signatures. Fixed.
VerifyUserIdSignature was used for checking the self-signature on an identity,
but didn't support anything but a self-signature. This change alters the
low-level API so that checking cross-signatures is possible.
Fixesgolang/go#7371.
LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/96240043
Manual edits to README.
Moved from main Go repository, deleted Makefiles, ran gofix -r go1rename.
Tested with: go test code.google.com/p/go.crypto/...
R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5564059