Async publishing and API proposals

[mtmk]

See updated readme here https://github.com/nats-io/nats.swift/tree/fix-async-pub

While updating the examples I made some changes to the API to make is a little more dev friendly:

• made publish async
• publish subject is named as to: to match subscribe()
• changed how options are accepted moving away from builder (this is experimental)
• added easier to use subscripts for headers
• updated readme and examples