Restructure into apps, libs, settings structure

Based on this [Heroku deploying django best practices guide](http://www.deploydjango.com/django_project_structure/) I am restructuring the application.

@ukanga @larryweya please take a look and the guide and confirm this is OK ASAP.  Here is a diff from the other PR, https://github.com/onaio/onadata/compare/pld/add-json-column...pld/apps-lib-structure

(It is in progress so failing tests for now.  I wanted to get feedback so opened the PR.)