Adds valid HTTP/1.1 default response headers

- Adds new dependency: `Calendar`
- RFC1123 conformant datetime formatter
- Passes httplint with 0 warnings

Closes #10

```.jl
Checking URL http://localhost:8000/README.md
* HTTP/1.1 200 OK [0d][0a]
    A response status code in the range 200 - 299 indicates that the
    request was successful.

* Content-type: text/html; charset=UTF-8[0d][0a]
    OK.

* Server: Julia/0.2.0-766.rfe3c Meddle/0.0[0d][0a]
    OK.

* Date: Wed, 27 Mar 2013 08:41:49 GMT[0d][0a]
    OK.

* Content-language: en[0d][0a]
    OK.

* [0d][0a]
    End of headers.


    No Last-Modified header was present. The HTTP/1.1 specification states
    that this header should be sent whenever feasible.
```