title: Trouble finding certain tag values
body: <p>I'm trying to find the value of several tags using pydicom. For some reason only certain tags work while others do not. Below is a traceback explaining my problem. Can anyone see a way around the int() base 16 problem?</p>
  
  <pre><code>&gt;&gt;&gt; ds['0x18','0x21'].value
  'SP'
  &gt;&gt;&gt; ds['0x18','13x14'].value
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/space/jazz/1/users/gwarner/anaconda/lib/python2.7/site-packages/pydicom-0.9.9-py2.7.egg/dicom/dataset.py", line 276, in __getitem__
  tag = Tag(key)
    File "/space/jazz/1/users/gwarner/anaconda/lib/python2.7/site-packages/pydicom-0.9.9-py2.7.egg/dicom/tag.py", line 27, in Tag
  arg = (int(arg[0], 16), int(arg[1], 16))
  ValueError: invalid literal for int() with base 16: '13x14'
  </code></pre>