Improved plot digits classification example.

Made two for loop statements more readable by introducing local variables.
<MRG> Doc generation works in Python 2 and 3

Documentation can be generated using Python 2 or Python 3 and sphinx
FIX Move pooling_func to constructor.
[MRG] Addressed comments
FIX: AdditiveChi2Sample can be initialized with sample_interval, #3068
[MRG] Addressed comment, fixed open-ended except

Except wile opening a URL now only catches AttributeError, eliminating an issue when
running Python 2 without internet connection.
More Py3 fixes for sphinx build
Merge branch 'python-3-sphinx-fixes'
Merge pull request #3063 from chalmerlowe/improved-digits-example

[MRG+1] Improved plot digits classification example.
added tests for sparse matrix inputs to BaggingClassifier and BaggingRegressor
open iris files with  statement to avoid ResourceWarning
FIX: One more PY3 fix in the documentation generator
Add tests for the sample_interval in #3069
plot_adaboost_multiclass.py: handle case where boosting terminated early. Add missing author on other boosting examples.
xrange -> range
Merge pull request #3078 from ndawe/examples

[MRG] plot_adaboost_multiclass.py: handle case where boosting terminated early
Merge pull request #3074 from msalahi/iris-file-warnings

open iris files with 'with' statement to avoid ResourceWarning
FIX: typo in gen_rst.py
FIX: More robust test_extmath.py, compatible with older numpy
Fix the pep8 violations