platforms/firmata/client: use io.ReadFull
platforms/firmata/client: use io.ReadFull

This CL uses io.ReadFull to make sure exactly n bytes are read, and
correctly handle io.EOF and co.

Fixes #343.

Signed-off-by: Sebastien Binet <seb.binet@gmail.com>

Conflicts:
	platforms/firmata/client/client.go

nzmichaelh/gobot,nzmichaelh/gobot