Add a bufio buffer to rpc/bsonrpc.

The following patch speeds up bsonrpc by adding bufio to the encoder and decoder.

Pre Bufio:
BenchmarkEndToEndBSON-2	   50000	     52024 ns/op

With Patch:
BenchmarkEndToEndBSON-2	   50000	     37934 ns/op

For more see: https://gist.github.com/pcdummy/6168792