Add conversion to binary for Publish message contents

[mjkapkan]

Had a need to send binary content so added sort of a workaround that converts a hex string in to bytes. Ideally it should detect JS type automatically, but this is the first time for me with GO so did not want to go too deep. Therefore if someone has better suggestions on how to achieve it, I'm all in. This is an attempt to fix issue: #2 .

[pmalhaire]

I could not find an easy way to convert js binary array to golang so I am closing this as a limitation for now