Make epydoc work with docutils-0.6.

Patch created by Engelbert Gruber (grubert) and adjusted by Martin von Gagern.

References:
https://sourceforge.net/tracker/?func=detail&aid=2895197&group_id=32455&atid=405618
http://thread.gmane.org/gmane.text.docutils.devel/4888/focus=4924
http://bugs.gentoo.org/287546

Index: epydoc-3.0.1/epydoc/markup/restructuredtext.py
===================================================================
--- epydoc-3.0.1.orig/epydoc/markup/restructuredtext.py
+++ epydoc-3.0.1/epydoc/markup/restructuredtext.py
@@ -304,10 +304,11 @@ class _SummaryExtractor(NodeVisitor):
         # Extract the first sentence.
         for child in node:
             if isinstance(child, docutils.nodes.Text):
-                m = self._SUMMARY_RE.match(child.data)
+                data = child.astext()
+                m = self._SUMMARY_RE.match(data)
                 if m:
                     summary_pieces.append(docutils.nodes.Text(m.group(1)))
-                    other = child.data[m.end():]
+                    other = data[m.end():]
                     if other and not other.isspace():
                         self.other_docs = True
                     break