Minimal spanning tree backported from scipy 0.13

No tests in this commit
Added test
Moved mst to a subfolder and added a README file
Added new files (from previous commit)
Support both masks and arrays of indices.
shorten displayed dosctring to 95 chars
Typo.
DOC Detail on parent-child relationship in tree
Merge pull request #2031 from jnothman/tree_comments

DOC Detail on parent-child relationship in tree
Rename _apply_transform to _transform_selected and make it a function
rather than a method.
FIX #2032, FIX #2033: ensure module names consistency with __all__
Remove redundant test that was checked in by mistake
fix white space inconsistency between header and docstring
Merge pull request #1914 from fabianp/ridge_svd

MRG - Add SVD-based solver to ridge regression.
example docstring fixes
logistic regresion example fix
FIX/COSMIT helper to identify target types

* Fixes bugs related to simply checking type(..) equality
* Refactors metrics and preprocessing
* Invalidates mix of sequences of sequences and label indicators in
  metrics
* Invalidates row vectors in metrics
* Fixes bug where single-class target led to incorrect F-score
FIX cannot use set notation for Py2.6
FIX need explicit dtype for array of sequences in numpy 1.3
COSMIT remove redundant target size check

This error is already being raised by check_arrays