dpll.js: Manipulate search tree inside a function

This cleans the code a little bit

Signed-off-by: Diego Giovane Pasqualin <dpasqualin@c3sl.ufpr.br>
dpll.html: Remove elapsed time

Signed-off-by: Diego Giovane Pasqualin <dpasqualin@c3sl.ufpr.br>
Allow user to advance on execution through steps

There are still some things to fix and improve but it seems to be working
nice.

Signed-off-by: Diego Giovane Pasqualin <dpasqualin@c3sl.ufpr.br>