Added metadata.xml file for epub/mobi.

Kindlegen requires title and author metadata.  To generate a mobi file from
a pandoc ebook, the pandoc ebook has to have that metadata via either code like
this at the top:

```
% Title
% Author
```

or via an external metadata file.  In order to keep the markdown clean, I added
as a separate file.  Ebooks w/ metadata can then be created via the command

```
pandoc -f markdown -t epub --epub-metadata=metadata.xml -o backbone-fundamnetals.epub README.md
```

Author: Ethan Winn

metadata.xml

@@ -0,0 +1,4 @@
+<dc:title>Developing Backbone.js Applications</dc:title>
+<dc:creator>Addy Osmani</dc:creator>
+<dc:rights>Creative Commons Attribution Non-Commercial Share Alike 3.0</dc:rights>
+<dc:language>en-US</dc:language>