IPFS 0.11.0 Support

[Arlodotexe]

A few months ago, IPFS released 0.11.0, which among some good improvements, also included breaking changes to the pubsub APIs.

Would be great to see this library brought up to speed with this version of IPFS. Willing to assist as needed, so long as the repo author is still around.

[Arlodotexe]

Some progress made in #70, and merged to master on my fork.

[Arlodotexe]

I'm working with the IPFS team to get this repo imported into the IPFS shipyard so the community can maintain it again.

One of the first things we'll be doing is consolidating our TargetFramework to netstandard2.0. There's a few reasons for this:

• net45 has since reach end of life
• netstandard1.4 is starting to show its age (also just not recommended anymore)
• As pointed out in Updated PubSub to align with 0.11.0  #70, the cs-multibase library recommended by IPFS only supports netstandard2.0. This is a complex thing to create and is becoming an increasingly important part of the spec, so we're bringing this dependency along.