server: accept iolists from protocol buffers encoder (fixes #3)

A recent change in erlang_protobufs caused the encoder functions to
return iolists instead of plain binaries. While this makes sense
performance wise it caused crashes in the server code when RPC packets
were assembled.
server: use tags for dependcies

Using branches as version specifier of dependencies is a bad idea as
it's inherently volatile and might break the build unexpectedly.