Changed default in NiftiMasker

Changed "standardize" keyword argument from False to True.
Created "extracting data" section.

Moved description of NiftiMasker from section "fMRI data manipulation"
to the new section. Adapted the text as well.
Written some doc on Nifti(Maps|Labels)Masker
Fixed docstrings in resampling.py
Remove spurious non-breakable spaces.
Completed list of functions in classes.rst

Also added some links, removed broken ones.
Added a lot of links in extraction_data.rst
Added links to examples, reformulated.