Use cached value for number of project tasks
Added project_tasks method to cached apps
Tests for new cache function
Use cached tasks in tasks/browse
Updated default theme
Merge branch 'issue-query-perf-improvements' of https://github.com/PyBossa/pybossa into issue-query-perf-improvements

* 'issue-query-perf-improvements' of https://github.com/PyBossa/pybossa:
  use app.id instead of app_id for tasks. app_id could be null