Definition of branch, branch point, branch cuts...

Try to make the goal easier to understand and the definitions (a bit) more
rigorous.
Example for branches

function sqrt(z*(z-1))

Correction of a bug (not shown before ?) in inclusion of
riemann_surface_log.jpg.
Address comments of Gp2mv3/Syntheses#117

+ Label position
+ Explaination of "feuille"
Correction of details
Definition of the integral of a complex function.
Merge pull request #117 from cassiersg/math3

Math3 : complexes