yubikey-agent: unescape pinentry output

Fixes FiloSottile#61

all: switch to golang.org/x/term

README: update Linux installation instructions

all: update piv-go dependency

Fixes FiloSottile#34
Fixes FiloSottile#32 (maybe)
Fixes FiloSottile#14
Fixes FiloSottile#13

README: populate most docs

yubikey-agent: switch to P-256 ECDSA