ENH: add single_linkage
ENH: single linkage with Cython

Faster thanks to a binding of the C++ map container to replace Python
dictionaries
ENH: speed up single linkage
ENH: start of complete linkage
ENH: first sketch of weighted linkage
BUG: fix bug introduced in Ward
ENH: add average linkage
BUG: remove debug print
WIP: MST for single linkage
Cosmit
MISC: refactor Ward and linkage in same object
ENH: hierarchical clustering example
DOC: nice hierarchical clustering example
API: rename to agglomerative clustering
Add cython code
MISC: manual merge of master
MISC: Take in account @NelleV's comments
FIX spelling mistakes and whitespace nitpick
PEP8 fixes on hierarchical.py
ENH improved the documentation of the fix_connectivity function