epkowa: add the GT-S650 plugin

This plugin adds support for the GT-S650, Perfection V19 and
Perfection V39 scanners. I've tested it personally with an Epson
Perfection V39.

This plugin uses the ISCAN_FIRMWARE_DIR environment variable, not
ISCAN_FW_DIR. A quick grep didn't find any use of ISCAN_FW_DIR in the
currently packaged plugins, but a quick Google suggests that this
variable does (or at least used to) exist, so I'm not touching it.