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.
Renamed searchlight.py to decoding.py

This is the first step toward a module dedicated to decoding algorithms.
Added decoding module to documentation.
Corrected some Sphinx compilation errors
Created directory "decoding"
Moved searchlight tests in decoding/tests
Added a comment in plot_simulated_data.py
Fixed examples

Some examples were broken after changing the default value for "standardize"
in NiftiMasker class.