Adds anonymous logon

[crossworth]

I'm not sure if I'm building the logOn message correctly yet, it logons on Steam without problems, but maybe something is missing, I need do more testing before it can be merged.

In the mean while I have a question:

On the Node library the SteamID Instance for the anonymous account is ALL where in this library is Unknown, on the C# library it's All as well, maybe it sould be called AllInstance to match the others libraries?

https://github.com/DoctorMcKay/node-steamid/blob/4cbc98c2723ea92eba021a937dd9c7ae033ebe3f/index.js#L31
https://github.com/SteamRE/SteamKit/blob/master/SteamKit2/SteamKit2/Types/SteamID.cs#L72

[crossworth]

Sorry about this change, Goland sort the imports by external then internal packages...

[crossworth]

I have tested this implementation on OSX, Linux and Windows and found no problems, I have checked the default ClientLogon message by steamcmd.exe and I think we are sending the proto correctly.

This is what SteamCMD sends on anonymous logon:

This PR is ready for review.

[crossworth]

After a few more "real world" tests I find that this implementation is not that good.

For example I need to specify the Cell ID while logging anonymously. We could ofcourse add an details param, but if we do that, would be better handle the in the LogOn method.

Maybe we could remove this checks:

if details.Username == "" {
    return errors.New("steam/auth: username must be set")
}

if details.Password == "" && details.LoginKey == "" {
    return errors.New("steam/auth: Password or LoginKey must be set")
}

And make the logOn more like the NodeJS implementation:
https://github.com/DoctorMcKay/node-steam-user/blob/master/components/logon.js#L204

I will leave this PR open since there is no issues enable on this repo, but fell free to close it.