fix spelleng mistaek
Merge pull request #610 from onaio/609_dox_typo_fix

fix spelleng mistaek (issue #609)
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