Use HKDEBUG to dump full requests and responses.

Replaces HKDUMPREQ.

```
$ HKDEBUG=1 ./hk create omgwtfbbq
POST /apps HTTP/1.1
Host: api.heroku.com
User-Agent: hk dev darwin amd64
Content-Length: 20
Accept: application/vnd.heroku+json; version=3
Authorization: Basic CREDS
Content-Type: application/json
Accept-Encoding: gzip

{"name":"omgwtfbbq"}
HTTP/1.1 422 Unprocessable Entity
Content-Length: -1
Connection: keep-alive
Content-Type: application/json;charset=utf-8
Date: Wed, 16 Oct 2013 21:28:35 GMT
Oauth-Scope: global
Oauth-Scope-Accepted: global write write-protected
Ratelimit-Remaining: 2399
Request-Id: 367ea525-7a02-4e4f-9102-abcd12345678
Server: nginx/1.2.8
Status: 422 Unprocessable Entity
Vary: Accept-Encoding
X-Content-Type-Options: nosniff
X-Runtime: 0.066401904

{"id":"invalid_params","message":"Name is already taken"}
Unexpected error: 422 Unprocessable Entity
```

Thinking about adding spacing or a separator between the request & response for better readability.