Merge pull request #2779 from jnothman/ref_sec_link

[MRG] DOC add link to module reference from class/function pages
Update plot_digits_pipe.py

Import statements have been moved to the top of the file.
Merge pull request #2780 from ugurthemaster/patch-1

Update plot_digits_pipe.py
DOC: some underlines were too short
FIX replace 'f1' scorer by explicit variants

This makes the averaging options clearer for users

It entails a deprecation procedure for scorers.
DOC add scoring changes to what's new
ENH add scorers for samples averaging
FIX Require explicit average arg for multiclass/label P/R/F metrics
COSMIT pep8 fixes
COSMIT prefer explicit set method over operators
COSMIT tidy up scorer creation