Fixed #902: a bug of code block caused by span tags that cross multiple lines.

This patch makes sure all tags are enclosed in every line with the
correct syntax class.

Input:
``` java
/**
 * Establishes a contract for reading events stored in arbitrary formats from
 * reliable, resettable streams.
 */
```
Original output:
``` html
<div class="line"><span class="javadoc">/**</div>
<div class="line"> * Establishes a contract for reading events stored in arbitrary formats from</div>
<div class="line"> * reliable, resettable streams.</div>
<div class="line"> */</span></div>
```
With this patch:
``` html
<div class="line"><span class="javadoc">/**</span></div>
<div class="line"><span class="javadoc"> * Establishes a contract for reading events stored in arbitrary formats from</span></div>
<div class="line"><span class="javadoc"> * reliable, resettable streams.</span></div>
<div class="line"><span class="javadoc"> */</span></div>
```