Update XML so it _elem is no longer undefined

When I went to use your plugin on my server it failed because XML was outdated. The error:
```
TypeError: Cannot read property '_elem' of undefined
  at resolve (/Users/SJAnderson/dev/SJAnderson/node-podcast/node_modules/xml/lib/xml.js:143:19)
  at /Users/SJAnderson/dev/SJAnderson/node-podcast/node_modules/xml/lib/xml.js:189:42
  at Array.forEach (native)
  at resolve (/Users/SJAnderson/dev/SJAnderson/node-podcast/node_modules/xml/lib/xml.js:182:24)
  at /Users/SJAnderson/dev/SJAnderson/node-podcast/node_modules/xml/lib/xml.js:189:42
  at Array.forEach (native)
  at resolve (/Users/SJAnderson/dev/SJAnderson/node-podcast/node_modules/xml/lib/xml.js:182:24)
  at add (/Users/SJAnderson/dev/SJAnderson/node-podcast/node_modules/xml/lib/xml.js:47:24)
  at xml (/Users/SJAnderson/dev/SJAnderson/node-podcast/node_modules/xml/lib/xml.js:88:9)
  at Podcast.xml (/Users/SJAnderson/dev/SJAnderson/node-podcast/lib/podcast.js:67:19)
  at Object.<anonymous> (/Users/SJAnderson/dev/SJAnderson/node-podcast/testing.coffee:56:18)
  at Object.<anonymous> (/Users/SJAnderson/dev/SJAnderson/node-podcast/testing.coffee:1:1)
  at Module._compile (module.js:456:26)
```

Line 143 of xml.js@~0.0.4, your version
```js
        if (values._elem) {
            values._elem.name = name;
            values._elem.icount = indent_count;
            values._elem.indent = indent;
            values._elem.indents = indent_spaces;
            values._elem.interrupt = values;
            return values._elem;
        }
```

The latest version of xml addresses this problem by adding the following conditional
`if (values && values._elem) {`
 
When I update XML to the latest version, 1.0.0, it now works.

```html
<?xml version="1.0" encoding="UTF-8"?>
<rss xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:content="http://purl.org/rss/1.0/modules/content/" xmlns:atom="http://www.w3.org/2005/Atom" xmlns:itunes="http://www.itunes.com/dtds/podcast-1.0.dtd" version="2.0" xmlns:geo="http://www.w3.org/2003/01/geo/wgs84_pos#">
  <channel>
    <title><![CDATA[title]]></title>
    <description><![CDATA[description]]></description>
    <link>http://example.com</link>
    <image>
      <url>http://example.com/icon.png</url>
      <title>title</title>
      <link>http://example.com</link>
    </image>
    <generator>Podcast for Node</generator>
    <lastBuildDate>Sat, 27 Sep 2014 00:14:02 GMT</lastBuildDate>
    <atom:link href="http://example.com/rss.xml" rel="self" type="application/rss+xml"/>
    <author><![CDATA[Dylan Greene]]></author>
    <pubDate>Sun, 20 May 2012 04:00:00 GMT</pubDate>
    <copyright><![CDATA[2013 Dylan Greene]]></copyright>
    <language><![CDATA[en]]></language>
    <managingEditor><![CDATA[Dylan Greene]]></managingEditor>
    <webMaster><![CDATA[Dylan Greene]]></webMaster>
    <docs>http://example.com/rss/docs.html</docs>
    <ttl>60</ttl>
    <itunes:author>Max Nowack</itunes:author>
    <itunes:subtitle>I am a sub title</itunes:subtitle>
    <itunes:summary>I am a summary</itunes:summary>
    <itunes:image href="http://link.to/image.png"/>
    <itunes:explicit>No</itunes:explicit>
    <itunes:owner>
      <itunes:name>Max Nowack</itunes:name>
      <itunes:email>max@unsou.de</itunes:email>
    </itunes:owner>

    <category><![CDATA[Category 1]]></category>
    <category><![CDATA[Category 2]]></category>
    <category><![CDATA[Category 3]]></category>
    <item>
      <title><![CDATA[item title]]></title>
      <description><![CDATA[use this for the content. It can include html.]]></description>
      <link>http://example.com/article4?this&amp;that</link>
      <guid isPermaLink="false">1123</guid>
      <dc:creator><![CDATA[Guest Author]]></dc:creator>
      <pubDate>Sun, 27 May 2012 07:00:00 GMT</pubDate>
      <itunes:author>Max Nowack</itunes:author>
      <itunes:explicit>No</itunes:explicit>
      <itunes:subtitle>I am a sub title</itunes:subtitle>
      <itunes:summary>I am a summary</itunes:summary>
      <itunes:duration>12345</itunes:duration>
      <itunes:keywords>podcast</itunes:keywords>
      <geo:lat>33.417974</geo:lat>
      <geo:long>-111.933231</geo:long>
    </item>
  </channel>
</rss>
```