Fix rendering for 500 page error

Fixes pybossa/pybossa#733
Fix broken url_for
Merge pull request #16 from PyBossa/issue-720-pybossa-app-factories

Issue 720 pybossa app factories
Merge pull request #17 from PyBossa/issue-733-500-page-rendering

Fix rendering for 500 page error
Quickfix for SF logo like on pybossa repo (exceptional small direct master commit)
Merge branch 'master' into issue-687-app-blogs

* master:
  Quickfix for SF logo like on pybossa repo (exceptional small direct master commit)
  Fix broken url_for
  Fix rendering for 500 page error
Small improvements in blog post template