Added support for Protobuf codec and moved RPC system to it.

The support is a modified version of the protorpc package at:
http://godoc.org/code.google.com/p/protorpc.

Our version is modified to support gogoprotobuf. Eventually we'd
like to remove the rpc/codec subpackage and use gogoprotobuf's
implementation. I've started a thread with them for support.
Added tests and updated TODO.md
update godeps with snappy
Added generated .pb.go files
fix race in rpc heartbeat test
Merge pull request #165 from cockroachdb/spencerkimball/proto-codec

Added support for Protobuf codec and moved RPC system to it.