UK: return media storage url, which will most likely not require authentication or include authentication info in the url. closes #624
Merge pull request #626 from onaio/624_aws_links

return download_url to images that does not require auth
DW: Prepares onadata for the python json xls lib which will be added
DW: Added the python-json2xls lib and the test cases
DW: Replaces the / with _ on the field names
DW: Corrected the failing test
Dw: Added external_export in metadata and stores the external server url
DW: Query the data from postgres and not from mongodb
Dw: Code cleanup
DW: Can use either token or the token stored in the metadata
Initial ui changes
DW: add external export ui stuff
Dw: added a simple view to add and upload a template to the metadata
DW: resolved the failling tests
DW: Change the button labels for external export