Export as an attachment.
Merge branch 'master' of github.com:PyBossa/pybossa into issue-export-csv-attachment
Fixes for CSV export
Export task and task_run in CSV with all its members.

This commit only explodes first level of info.
Export JSON as an attachment.
Remove raise, and return 404 for unknown args
Fix to include 0 as a string in exporting as CSV
Tests for export CSV
Merge branch 'master' into issue-export-csv-attachment
Tests for CSV task run export
Download it as a file.
Merge pull request #838 from PyBossa/issue-export-csv-attachment

Issue export csv attachment