Revert "libertree-backend package should own its directory"

This reverts commit 7446d147d58394ecf6efc49374ba2a1782db95fa.
build rdbi gem/deb/rpm only when necessary
build rdbi-driver-postgresql only when necessary
simplify
prefix dependencies but not generated package
remove pony
remove syck
add mail-gpg/gpgme dependency
variable definition may not have trailing chars
add ruby-oembed dependency (for libertree-model)
add gpgme as a dependency for gpgme gem
fix package names to libertree-{backend,model,client}