Merge pull request #522 from PyBossa/issue-517-categories

Issue 517 categories
Fix n_count SQL query to count items. Closes #517
Docs for Categories
Compare values not identities
Refactor app2_index to app_cat_index
Fix try except error
Fix try except error
Manage try except for all calls
Add authz for users too
Check for authz in users
Return proper error messages
Remove print method
Custom 500 error page