Adding a new API endpoint

New API endpoints should use the v2 format. See examples of other v2 endpoints for how to handle this.

Checklist for adding a new API endpoint:

• Choose an existing, or create a new API set (see below for how to add an API set)
• Register it in api/http.go
• Add it to the list of endpoints in api/http_test.go
• Write unit tests for it in the relevant api/*_test.go file. Tests must follow the example of existing tests.
• Add a method to the api.Client in api/client.go
• Add a stable integration test, if applicable
• Add a live integration test, if applicable
• Update api/README.md
• Update CHANGELOG.md

Adding an API set:

• Define new API set name in api/http.go
• Add to allAPISetsEnabled in api/http_test.go
• Add to allAPISets (two instances) in skycoin/config.go
• Update documentation for the CLI options in skycoin/config.go
• Update documentation of API sets in api/README.md