title: Place link label in the middle of a link in JoinJS
body: <p>For some reason, when I'm creating a link, in the function <code>updateLabelPositions</code> of  <code>joint.dia.LinkView</code>, I get <code>NaN</code> when the following line is executed:
  <code>var connectionLength = connectionElement.getTotalLength();</code></p>
  
  <p>This causes the code to fail later on when calculating the label position.<br>
  I'm using <em>joint.clean.js v0.9.0</em> + <em>Backbone 1.0.0</em>.<br>
  This issue occurs only in Firefox 30.0, while in Chrome Version 35.0.xxx... and IE10 it's all fine.</p>
  
  <p>Here is my link creation code:</p>
  
  <pre class="lang-js prettyprint-override"><code>var link = new joint.dia.Link({
                  source: { id: sourceNodeLabel },
                  target: { id: targetNodeLabel },
                  smooth: true
              });
  
  link.label(0, { position: .5, attrs: {text: { text: 'label', 'font-size': 14, 'font-family': 'san-serif' }}});
  </code></pre>
  
  <p>This code is taken from JoinJS demos: <a href="http://www.jointjs.com/demos/links" rel="nofollow">http://www.jointjs.com/demos/links</a>
  (link no.6  in their sample)</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>During investigation, I've found out that Mozilla has bug where the function <code>getTotalLength</code> of <code>SVGPathElement</code> returns <code>NaN</code>. Is anyone familiar with this issue maybe?<br>
  check this jsFiddle: <a href="http://jsfiddle.net/Xyk3R/" rel="nofollow">http://jsfiddle.net/Xyk3R/</a></p>
  
  <p>please let me know if there's any further information is needed.</p>
  
  <p>Cheers!</p>