Change class option to take strings and document it

[silverwind]

This changes to class option to take strings instead of integers, so 1 becomes 'IN', 3 becomes 'CH' and so on. Also documented the class option.

Two questions:

1. Should I reduce the documentation on the option? Given that most people will probably never need that option, we could remove it from the example. On the other hand, this is a low level library so it might just be okay to keep it.
2. This is a breaking change for a undocumented option. Should we still do a major version bump to be safe?

Fixes: #3

[silverwind]

I just read about the unicast-response bit that multicast dns encodes in the class bytes. I'm thinking that we could encode in a separate property.

[mafintosh]

👍 and 👍 to using a different prop for unicast

[mafintosh]

@silverwind should we merge this?

[silverwind]

No, it's not done yet, I'll check it out later. We should probably do a patch release before we land this, because this one will be major bump.

[silverwind]

Rebased, I will track the unicast-response thing in a separate issue.