Add -X to okcurl, permitting HEAD requests

Handy for testing spdy (https://google.ca) and http/2 (https://twitter.com)

Ex.
```bash
~/Development/okhttp/okcurl adrian.curl-X ./target/okcurl-2.0.0-SNAPSHOT-jar-with-dependencies.jar -X HEAD -i https://twitter.com
HTTP/1.1 200
OkHttp-Selected-Transport: HTTP-draft-09/2.0
OkHttp-Selected-Protocol: HTTP-draft-09/2.0
cache-control: no-cache, no-store, must-revalidate, pre-check=0, post-check=0
content-length: 51304
content-type: text/html;charset=utf-8
date: Tue, 11 Feb 2014 16:59:06 GMT
expires: Tue, 31 Mar 1981 05:00:00 GMT
last-modified: Tue, 11 Feb 2014 16:59:06 GMT
ms: S
pragma: no-cache
server: tfe
set-cookie: REDACTED
status: 200 OK
strict-transport-security: max-age=631138519
x-content-type-options: nosniff
x-frame-options: SAMEORIGIN
x-transaction: 81bc971b3d29aa7f
x-ua-compatible: IE=10,chrome=1
x-xss-protection: 1; mode=block
OkHttp-Sent-Millis: 1392137948438
OkHttp-Received-Millis: 1392137948528
OkHttp-Response-Source: NETWORK 200
```

cc @JakeWharton @jpinner