Provide generic logging interface

[andig]

Migrated from goburrow/modbus#33

[gq0]

Hi @andig, thanks for forwarding the PR.

Just to understand your intend first. You want to register the logrus Logger implementation at the library, correct? Would the following code snipped already be sufficient for your purpose?

handler := modbus.NewTCPClientHandler(address)
handler.Logger = log.New(logrus.New().Writer(), "your prefix ", log.LstdFlags) 

[andig]

Yes it would. Thats not necessary. You can use log or don't if you don't want it. Actually, its even simpler:

handler.Logger = logrus.New()

or whatever the client wants to do.

Or I could inject any other object that implements Printf. Without changing anything really its already decoupled from golang log implementation.

[gq0]

I see your point here. Only Printf is required across the repo.