dpll.js: Add comments to all functions

Signed-off-by: Diego Giovane Pasqualin <dpasqualin@c3sl.ufpr.br>
dpll.js: Reorder methods, public ones on top, privates on bottom

Signed-off-by: Diego Giovane Pasqualin <dpasqualin@c3sl.ufpr.br>
dpll.js: Fix formula shown on popup on Graph

- Shows [unsat clause] on unsat clauses.
- Correctly apply assignment on branches that are not search by the
  algorithm but we want to show on the search tree

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