Implement task in our stylesheets

[tomschr]

The task element should be rendered correctly for HTML and PDF. Related to #94

[tomschr]

A first draft looks like this and is implemented in the feature/task branch at commit 2325041

The XML structure looks like this:

<para>The following is a task:</para>
 
<task xml:id="task.example">
  <title>An Example Task</title>
  <tasksummary>
   <para>This is the summary of a task. The first paragraph of the quick </para>
  </tasksummary>
  <taskprerequisites>
   <para>This contains the requirements</para>
  </taskprerequisites>
  <procedure>
   <title>Hopefully some useful steps</title>
   <step>
    <para>The first step</para>
   </step>
   <step>
    <para>The second step</para>
   </step>
  </procedure>
  <taskrelated>
   <para>Some taskrelated stuff</para>
  </taskrelated>
</task>

<para>This is something after the task. Maybe the quick brown fox jumps over the lazy dog?</para>

[tomschr]

Still relevant (also for SmartDocs). Need to discuss this further what we want and how we want it.