UK: specify string to use for empty values in csv exports, defaults to n\a
PLD: remove submodules, closes #96
Merge pull request #138 from onaio/remove-git-submodules-96

PLD: remove submodules, closes #96
Merge pull request #136 from kobotoolbox/na-values

Specify string to use for empty values in csv exports, defaults to n\a
use common settings as default to prevent local_settings from being overidden by default_settings
Merge pull request #139 from onaio/settings_restructure

use common settings as default to prevent local_settings from being overidden by default_settings