[major] f*** gpgme, use Go's own openpgp and Camlistore's gpgagent/pinentry instead

This PR removes all occurences of gpgme and its ugly cgo integration, and uses stock openpgp functions instead. It will remove a dependency, but also fix cross-compilation issues.
doesn't look like it does. Plus the mlock syscall in Go is only implemented for linux: http://golang.org/src/pkg/syscall/zsyscall_linux_amd64.go?s=29479:29511#L1302