New feature: more sourcecode highlighting

Hi!

I just added support (and dependency) for SilverCity to PyDS (Roberto Alsina
pointed me to it). This adds a new reST directive code-block that is used
like this:

#raw

.. code-block::

  #include <stdio.h>

  void main(int argc, char **argv) {
    printf("Hello World\n");
  }

#end raw

The #raw ... #end raw is needed because of the #include - otherwise the
cheetah engine would see that as a cheetah directive. It's wise to include
source blocks with #raw #endraw anyway, as there are many things in
languages that might blow cheetah (for example it get's quite excited about
perl comments and perl variables, too ;-)).

Since there are several programmers using PyDS, I (and obviously Roberto)
thought this might be usefull, even though it adds another dependency to
PyDS.

bye, Georg