ENH Ward: better behavior for non-fully-connected graphs

Improves the plot_cluster_comparison example
ENH: Don't modify connectivity unless specified
DOC: affinity-propagation in clustering comparison

Also add a 'null' example for clustering
DOC: add clustering example on front page
ENH: a few typos in docstrings
Merge pull request #726 from emmanuelle/doc_correction

ENH: a few typos in docstrings
ENH: summary table on clustering
DOC: better clustering comparison table
DOC clustering comparison: link table and figure
MISC: tweak example layout
DOC: finish table to compare clustering
Merge branch 'WIP_tut'
DOC: Better narrative for DBSCAN

Thanks to Robert Layton
DOC: finish misc in tutorial

@jaquesgrobler : there were still a few things to do :)
BUG: no plotting in doctests
COSMIT: layout tweak
Redo CSS layout killed by commut 94088b81
BUG: fix doctests
point out scalability problem with large numbers of classes;
use ``n_classes`` instead of ``K``
cosmit; mention scalability issues w.r.t. large number of classes