Unirest 5

[ryber]

I'm starting to build out some ideas for Unirest 5 and am open to ideas from the community on which direction we should go. Some of my goals are:

• No change to coordinates or namespaces, there will however be breaking changes to class signatures.
• clean up the interface for multipart form fields with a new builder for advanced options. This will be introduced in Unirest 4 but we will deprecate some of the more eccentric overloads and remove them in 5
• Support streaming mappers
• possibly offer new mapper modules? (protobuf)
• unify and refactor interceptor and metrics to be cleaner and simplified

Thoughts? Anything else?