Implement Atom feed

Feed format has been tested with curl:
```
$ curl -vH "Accept: application/atom+xml" http://localhost:3000/repos/mattmatt/hurt_logger/builds
* Adding handle: conn: 0x7f9b22004000
* Adding handle: send: 0
* Adding handle: recv: 0
* Curl_addHandleToPipeline: length: 1
* - Conn 0 (0x7f9b22004000) send_pipe: 1, recv_pipe: 0
* About to connect() to localhost port 3000 (#0)
*   Trying ::1...
*   Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 3000 (#0)
> GET /repos/mattmatt/hurt_logger/builds HTTP/1.1
> User-Agent: curl/7.30.0
> Host: localhost:3000
> Accept: application/atom+xml
>
< HTTP/1.1 200 OK
< Date: Mon, 11 Nov 2013 16:52:49 GMT
< Status: 200 OK
< Connection: close
< Content-Type: application/atom+xml;charset=utf-8
< Access-Control-Allow-Origin: *
< Access-Control-Allow-Credentials: true
< Access-Control-Expose-Headers: Content-Type, Cache-Control, Expires, Etag, Last-Modified
< X-Endpoint: Travis::Api::App::Endpoint::Repos
< X-Pattern: /:owner_name/:name/builds
< X-OAuth-Scopes: public
< X-Accepted-OAuth-Scopes: public
< Vary: Accept,Accept-Encoding
< Cache-Control: no-cache
< Content-Length: 3469
<
<?xml version="1.0" encoding="utf-8"?>

<feed xmlns="http://www.w3.org/2005/Atom">

  <title>mattmatt/hurt_logger Builds</title>
  <link href="http://localhost:3000/repos/mattmatt/hurt_logger/builds" type="application/atom+xml" rel = "self" />
  <id>urn:uuid:3caf5813-770e-424c-a8ed-acb9ce1c3157</id>
  <rights>Copyright (c) 2013 Travis CI GmbH</rights>
  <updated>2013-11-11T11:52:49-05:00</updated>


  <entry>
    <title>mattmatt/hurt_logger Build #11</title>
    <link href="https://travis-ci.org/mattmatt/hurt_logger/builds/791653" />
    <id>urn:uuid:c682b3e4-8ebd-43e1-a960-f223fc036674</id>
    <updated>2012-03-04 11:27:21 UTC</updated>
    <summary type="html">
    &lt;p&gt;
      Remove debug. (Mathias Meyer)
      &lt;br/&gt;&lt;br/&gt;
      State: passed
      &lt;br/&gt;
      Started at: 2012-03-04 11:25:48 UTC
      &lt;br/&gt;
      Finished at: 2012-03-04 11:27:21 UTC
    &lt;/p&gt;
    </summary>
    <author>
      <name>Mathias Meyer</name>
    </author>
  </entry>

  <entry>
    <title>mattmatt/hurt_logger Build #10</title>
    <link href="https://travis-ci.org/mattmatt/hurt_logger/builds/791645" />
    <id>urn:uuid:2094f189-b030-4653-925c-55e14960ca33</id>
    <updated>2012-03-04 11:24:55 UTC</updated>
    <summary type="html">
    &lt;p&gt;
      Tell what signal we've received. (Mathias Meyer)
      &lt;br/&gt;&lt;br/&gt;
      State: passed
      &lt;br/&gt;
      Started at: 2012-03-04 11:23:13 UTC
      &lt;br/&gt;
      Finished at: 2012-03-04 11:24:55 UTC
    &lt;/p&gt;
    </summary>
    <author>
      <name>Mathias Meyer</name>
    </author>
  </entry>

  <entry>
    <title>mattmatt/hurt_logger Build #9</title>
    <link href="https://travis-ci.org/mattmatt/hurt_logger/builds/791603" />
    <id>urn:uuid:ad475b3a-62a9-47ed-b29a-296a12c3fa13</id>
    <updated>2012-03-04 11:18:59 UTC</updated>
    <summary type="html">
    &lt;p&gt;
      Properly trap signal. (Mathias Meyer)
      &lt;br/&gt;&lt;br/&gt;
      State: passed
      &lt;br/&gt;
      Started at: 2012-03-04 11:17:35 UTC
      &lt;br/&gt;
      Finished at: 2012-03-04 11:18:59 UTC
    &lt;/p&gt;
    </summary>
    <author>
      <name>Mathias Meyer</name>
    </author>
  </entry>

  <entry>
    <title>mattmatt/hurt_logger Build #8</title>
    <link href="https://travis-ci.org/mattmatt/hurt_logger/builds/791325" />
    <id>urn:uuid:f5f10979-6792-4d81-acd7-82f5f0aade4f</id>
    <updated>2012-03-04 10:05:24 UTC</updated>
    <summary type="html">
    &lt;p&gt;
      Add tail command to display messages from pubsub drain. (Mathias Meyer)
      &lt;br/&gt;&lt;br/&gt;
      State: passed
      &lt;br/&gt;
      Started at: 2012-03-04 10:04:03 UTC
      &lt;br/&gt;
      Finished at: 2012-03-04 10:05:24 UTC
    &lt;/p&gt;
    </summary>
    <author>
      <name>Mathias Meyer</name>
    </author>
  </entry>

  <entry>
    <title>mattmatt/hurt_logger Build #7</title>
    <link href="https://travis-ci.org/mattmatt/hurt_logger/builds/791089" />
    <id>urn:uuid:40c521c5-887a-4185-ac4a-2e37ac8f194d</id>
    <updated>2012-03-04 09:05:14 UTC</updated>
    <summary type="html">
    &lt;p&gt;
      Read drains and filters from ENV. (Mathias Meyer)
      &lt;br/&gt;&lt;br/&gt;
      State: passed
      &lt;br/&gt;
      Started at: 2012-03-04 09:03:50 UTC
      &lt;br/&gt;
      Finished at: 2012-03-04 09:05:14 UTC
    &lt;/p&gt;
    </summary>
    <author>
      <name>Mathias Meyer</name>
    </author>
  </entry>


</feed>
* Closing connection 0
```

There are a few things I am not yet sure.

1. The conditions in `Travis::Api::App::Responders::Atom#apply?` may need some care
1. IDs in the Atom feed are of the form `urn:uuid:UUID`; is this correct?