add REST API functions and metatable for get

[wd60622]

Convenience wrapper around the REST API

local gh = require "octo.gh"

local opts = { cb = gh.create_callback() }

gh.api { -- or gh.api.get
  "/notifications",
  opts = opts,
}

This would be the same as gh api /notifications with the CLI

Other available methods are:

• gh.api.post
• gh.api.patch
• gh.api.delete

User can still specify the method if gh.api is used, e.g. gh.api { "/notifications", method = "GET" }

Closes #751