[#69] Add duration column
[#69] Add update duration function

* This time will be summed up to the existing duration
[#69] Fix duration update function

* Don't add the duration if the survey is already submitted
* Add duration field on Table creation
[#69] Store SurveyInstance session duration

* Store the session time in the database.
* Don't store the duration if we're in readOnly mode
* Save duration upon submission
[#69] Read survey duration form the database

* The value is already computed, just retrieve it
* Update DB queries for survey instance retrievals
[#70] Increase version number
Merge branch 'issue/69' into develop