epg: Utilize --pinentry-mode added in GnuPG 2.1

* epa.el (epa-pinentry-mode): New user option.
(epa-sign-file, epa-encrypt-file, epa-decrypt-region)
(epa-sign-region, epa-encrypt-region): Respect epa-pinentry-mode.
* epa-file.el (epa-file-insert-file-contents)
(epa-file-write-region): Respect epa-pinentry-mode.