xunit output doesn't handle unicode strings

xunit output throws an exception when some of the tracebacks have unicode strings in them.

The proposed fix interprets the traceback string as utf8 before writing it to a `minidom` attribute.

Example of failure:

```
Traceback (most recent call last):
  File "/Users/arnihermann/github/ja-neo/src/lettuce/lettuce/registry.py", line 88, in call_hook
    callback(*args, **kw)
  File "/Users/arnihermann/github/ja-neo/src/lettuce/lettuce/plugins/xunit_output.py", line 105, in output_xml
    wrt_output(output_filename, doc.toxml())
  File "/usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/dom/minidom.py", line 45, in toxml
    return self.toprettyxml("", "", encoding)
  File "/usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/dom/minidom.py", line 60, in toprettyxml
    return writer.getvalue()
  File "/usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/StringIO.py", line 271, in getvalue
    self.buf += ''.join(self.buflist)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 314: ordinal not in range(128)

Traceback (most recent call last):
  File "/Users/arnihermann/github/ja-neo/bin/lettuce", line 9, in <module>
    load_entry_point('lettuce==0.2.10', 'console_scripts', 'lettuce')()
  File "/Users/arnihermann/github/ja-neo/src/lettuce/lettuce/bin.py", line 88, in main
    result = runner.run()
  File "/Users/arnihermann/github/ja-neo/src/lettuce/lettuce/__init__.py", line 172, in run
    call_hook('after', 'all', total)
  File "/Users/arnihermann/github/ja-neo/src/lettuce/lettuce/registry.py", line 88, in call_hook
    callback(*args, **kw)
  File "/Users/arnihermann/github/ja-neo/src/lettuce/lettuce/plugins/xunit_output.py", line 105, in output_xml
    wrt_output(output_filename, doc.toxml())
  File "/usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/dom/minidom.py", line 45, in toxml
    return self.toprettyxml("", "", encoding)
  File "/usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/xml/dom/minidom.py", line 60, in toprettyxml
    return writer.getvalue()
  File "/usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/StringIO.py", line 271, in getvalue
    self.buf += ''.join(self.buflist)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 314: ordinal not in range(128)

```