RST style docstrings for Bio.Cluster (C code).

Tested using this, and then viewing the generated HTML:

    $ epydoc --version
    Epydoc, version 3.0.1
    $ epydoc -v -o ~/api -u http://biopython.org -n Biopython --docformat restructuredtext Bio/Cluster/

I'm not yet sure how to set the __docformat__ with a C module, but
hopefully before long we can just set the default to be RST when
we process everything (currently we use plain text as our default).