Refactor database access
Update taxonomy_dictionary to use the new DatabaseHelper.
Refactor 'Regions' model to use the new DatabaseHelper.
Refactor Countries and Country models to use the new DatabaseHelper.
Update Datasource and Datasources models to use the new DatabaseHelper.
Refactor Indicator and Indicators models to use the new DatabaseHelper.
Refactor Widgets model to use the new DatabaseHelper
Refactor the Detail model to use the new DatabaseHelper.
Remove unused comments
Remove unused comments in Datasources mode
Refactor AJAX helpers.
Remove unused files
Refactor models to use the CacheHelper.

All the models use now the CacheHelper to abstract the cache writing and reading.
This will lead to less bugs and more speed in the cache access.
Merge branch 'enhancement-#69' into develop

Close #69