Implement iterator multi.iperform and method easy.perform for easy.

```Lua
local cURL = require("cURL")
c1 = cURL.easy{url = "http://www.lua.org/"}
c2 = cURL.easy{url ="http://luajit.org/"}
m = cURL.multi()
  :add_handle(c1)
  :add_handle(c2)
for a,b,c in m:iperform() do ... end
```