remove trailing blanks in markdown files

We used this command to find markdown files with trailing whitespace.

```sh
find . -name '*.md' -print0 | xargs -0 grep -E -l '[[:space:]][[:space:]]*$'
```

After removal, we did a `make www`.

2012/hou/hint.html

@@ -120,12 +120,6 @@
 <!-- END: this line ends content from: inc/before-content.default.html -->
 <!-- START: this line starts content for HTML phase 21 by: bin/pandoc-wrapper.sh via bin/md2html.sh -->
 
-<h1 id="most-useful-obfuscation">Most useful obfuscation</h1>
-<p>Qiming Hou<br />
-<a href="mailto:hqm03ster@gmail.com"
-class="email">hqm03ster@gmail.com</a><br />
-<a href="http://www.houqiming.net"
-class="uri">http://www.houqiming.net</a></p>
 <h2 id="judges-comments">Judges’ comments:</h2>
 <h3 id="to-build">To build:</h3>
 <pre><code>make hou</code></pre>

2012/hou/hint.md

@@ -1,10 +1,3 @@
-# Most useful obfuscation
-
-Qiming Hou  
-<hqm03ster@gmail.com>  
-<http://www.houqiming.net>  
-
-
 ## Judges' comments:
 
 ### To build: