DOC: fix typos

s/nn/n/ in dictionary and optional
typo: s/accurracy/accuracy/g
Merge branch 'master' of github.com:scikit-learn/scikit-learn into enh/tree
Merge branch 'enh/tree' of github.com:bdholt1/scikit-learn into enh/tree
DOC small stuff in base.py and multiclass.py
Add mldata loader and olivetti dataset to changelog.

Thanks @ogrisel for noticing.
MSE bugfix (MSE.eval used to weight variances by n_left and n_right).
take DTYPE from _tree extension module
fix: inc n_left, n_right before variance computation; hopefully the last bugfix for MSE...
fix doctest + recompile cython code (accident)
Merge pull request #16 from pprett/bdholt1-enh/tree

Bdholt1 enh/tree
use np.isfortran
use None as the default marker
compute node id's on the fly
removed leftover class_counter
trees: don't use deprecated cross_val, error messages, use super
Merge pull request #17 from larsmans/enh/tree

trees: don't use deprecated cross_val, error messages, use super
added test for pickle-ability
Merge branch 'enh/tree' of github.com:bdholt1/scikit-learn into enh/tree
make Node an extension type + change class label indexing.