Use joblib from sklean.externals in example

plot_adhd_covariance.py required joblib to be installed
Move nilearn examples to dedicated subfolder
Tweak examples README.txt
Merge pull request #303 from lesteve/move-examples-to-subfolder

Move examples to dedicated 'examples' subfolder
Add glass brain plot to website carousel
Merge pull request #304 from lesteve/add-glass-brain-to-carousel

Add glass brain plot to website carousel
Merge branch 'master' into sparse-models
NF: poldrack loader (local)