Make the result of duneproject more like what is described in the

buildsystem howto.  This saves a lot of the copy&paste action
described there.  In particular:
- there is now a src/ subdirectory with Makefile.am, and the
  helloworld.cc is moved there.
- There is now a doc/ subdirectory with Makefile.am.
  The doxygen part is still missing, though.
- There is now a subdirectory named as the project itself,
  for header files which should be visible from the outside.


[[Imported from SVN: r5494]]