Changed functions to not return HTML strings

- Changed generate_alpha_rarefaction_data_from_point_in_omega to
return alpha rarefaction data instead of returning an HTML string. This
change was required to cope with the session management in
www/handler.psp.
- The tests suite was updated for this module.
Added Ids for iterations and visualizations
Changed alpha rarefactions to make use of session

- Multiple rarefactions and alpha diversity computations now take place
in lib.psp, hence if the user requires visualizations with stddev and
with stderr the same data will be used for both.
- handler.psp was updated to follow with the changes done to lib.psp
and to evident.rarefaction
Changed documentation as suggested in pull request
Merge pull request #25 from ElDeveloper/alpha_sessions

Integrated the use of session with alpha diversity