cleaner API, more efficient parsing

[slact]

      local SSE = require "resty.sse"

      local headers = {
        ["X-Some-Header-Field"] = "foobar"
      }
      local sse = SSE.new("http://some-pub-sub-server.com/subscriber", headers) --header table is optional
      --never fails, always returns new sse 'object'

      --explicitly calling :connect() is optional. SSE will connect as needed
      local ok, err = sse:connect()
      if not ok then 
        ngx.say("SSE failed: ", err)
        return
      end

      --message processing loop 
      for evt in sse:events() do
        ngx.say("SSE id:", evt.id or "")
        ngx.say("SSE event-type:", evt.type)
        ngx.say("SSE data:", evt.data)
      end
      
      --why did we exit the message processing loop?
      if sse.error then
        ngx.say("SSE error:", sse.error)
      end

don't forget to tag the merge commit 'v0.2.0' for luarocks