Smarter (import)

I'm a bit unhappy that hy has not one, but three import functions: `import`, `import-as` and `import-from`. We can surely do better than that! What I would love, is the following syntax:

```clojure
(import sys os os.path) ;; => import sys, os, os.path
(import [sys [exit argv]] [os [chdir]])
  ;; => from sys import exit, argv
  ;;    from os import chdir
(import [sys :as real_sys]) => import sys as real_sys
```

This is fairly straightforward to implement, as far as I see, especially now that we have keywords.