Change the data type of HTTP headers in ENV

ref #79 

* Change the structure of ENV
  * HTTP headers are moved into `:headers` in ENV
* The data type of headers is hash-table

```common-lisp
;; Before
(getf env :http-foo)

;; After
(gethash "foo" (getf env :headers))
```

**NOTICE**: This change breaks backward-compatibility.

Although I'm gonna check all open-sourced products to follow this change, if you have a private project using Clack, please check [change-headers](https://github.com/fukamachi/clack/tree/change-headers) branch.

Let me know if anybody have some problems in this.

And I deleted Clack.Handler.Apache because the project mod_lisp2 is abandoned and I don't recommend to use it.