Fix an error using session only in respond
Use session instead of engine.
Use db.session or session properly.
Fix session.execute format.
Merge pull request #864 from PyBossa/issue-update-task

Issue update task
Use update instead of save
Improvement in task repository delete
Merge branch 'master' into issue-repositories

* master:
  Fix session.execute format.
  Use db.session or session properly.
  Use session instead of engine.
  Fix an error using session only in respond

Conflicts:
	pybossa/cache/users.py
	pybossa/stats.py
	pybossa/view/applications.py
Update default theme