fix defaultdict call

In Python 2.7, the current version fails with:

$ python -c 'from collections import defaultdict; defaultdict(None)'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: first argument must be callable
MAINT deprecate sequences of sequences support

 * add warnings
 * provide alternative binarizer, sklearn.preprocessing.MultiLabelBinarizer
 * fix documentation
FIX remove duplicates in MultiLabelBinarizer
COSMIT use MultiLabelBinarizer in LabelBinarizer during deprecation
COSMIT use mlb for MultiLabelBinarizer instances

This class had been called LabelSetsBinarizer, for which lsb was appropriate
ENH do not allocate memory for temporary array of 1s
DOC remove mention of sequence of sequences in Parameters sections
FIX don't use dict comprehension for Py 2.6
FIX No set construction shorthand in Py2.6
DOC comment on _transform interface

Also avoid stride trick
TST Validate input MultiLabelBinarizer.inverse_transform
No set construction shorthand in Py2.6
DOC/FIX Address @arjoly's comments
TST stronger test for non-integers in MultiLabelBinarizer
Assert or ignore all sequence of sequences deprecation warnings
TST avoid more warnings related to sequence of sequences
TST fix testing for sequence of sequences warning in metrics
ENH use the np.int dtype to encode integer classes
FIX: Use coordinate_descent_gram when precompute is True | auto
Remove unused param precompute from MultiTask models