[#92] Delete empty Responses

* Automatically detect SurveyInstances without answers, and delete them
* close DB on onPause() callback
[#93] Delete empty Records

* Check for Records without SurveyInstances, and delete them
[#94] Force users to be logged in

* Move user check to SurveyGroupListActivity
* Note: Keeping track of User object within the RecordActivity is not
necessary anymore, as SurveyViewActivity can get the reference itself.
This approach will be implemented in the redesign branch (in the new Activity
to display Surveys)
[#95] Change 'Save and start new' to 'Save'

* Ensure each time the survey is saves, the status is set to SAVED
* Once the survey is submitted make it immutable (read only)