Add GitTip button to front page

I am not happy with how this looks, but the whole page needs a redesign
soon, so I will just be satisfied for the moment that the button is
now there.

src/ephem/doc/index.rst

@@ -19,6 +19,19 @@
      Data Sources<br/>
    </ul>
 
+   <p>
+     <i>Your contribution
+     <br>
+     supports PyEphem
+     <br>
+     development:</i>
+   </p>
+   <p>
+     <iframe style="border: 0; margin: 0; padding: 0;"
+             src="https://www.gittip.com/brandon-rhodes/widget.html"
+             width="48pt" height="20pt"></iframe>
+   </p>
+
    </td>
    <td>