Expose more OPC internals

[JoakimSoderberg]

Hello,

Nice implementation of OPC. However I quickly realised that I can't use it as is, since it is tailored quite specifically to one use case.

I wanted to write an OPC proxy, and that means I need raw access to the Message struct and so on. But as the current design is these are not exposed.

I did some changes to the library that allows me to just use some parts, and implement the connection/listening and so exactly as I want to.

This is my first attempt at a Golang project so I can't say I'm an expert. I'm totally open to modifying this PR if you want that, since I'm aware these break backwards compatability.

Some notable changes:

1. Made ReadOpc a standalone function. It did not use any of the state in the Server struct anyway. And this makes it possible to reuse in other code without being forced to use Server.

func ReadOpc(conn net.Conn) (*Message, error)

2. I made all fields in the Message struct exported. When I'm writing my proxy the Device concept does not really work, I just want to get the raw messages and send them on, and be able to check the channel and such for each message.

3. Renamed constants and changed comments to follow the "go standard" better. Examples:

SET_PIXEL_COLORS -> SetPixelColorsCmd

(Coming from C I kind of prefer your way as a matter of taste, but keeping to a language standard if there is such a clear one trumps that my personal taste any day).

Comments in Go should be of the form:

Something performs various actions
func Something()

Rather than:

This performs various actions
func Something()

This is from go vet suggestions.

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Anyway, what I ended up using of this is the changes to the exposed fields in Message as well as making ReadOpc a function that can be used standalone. So if you want me to modify the PR to only include those changes I can do that.