Speeds up level_list view of projects.

projects.children fetches those children from the DB. this caused the
retrieval of ~2000 projects to take upward of 10 seconds. Asking for
'leaf?' operates on nested set lft and rgt and does not require a
database query.
Merge pull request #49 from opf/fix/slow-level-list

Speeds up level_list view of projects.