User management interface
Fix issue 17: https://github.com/openwisp/OpenWISP-Geographic-Monitoring/issues/17
User CRUD interface now is accessible only to users which have the "wisp_viewer" role.
excel report saved in tmp and served through rails instead of being saved in the public folder
disable X-Sendfile in production environment; this allows us to avoid installing apache mod_xsendfile
added a basic test that checks whether the tmp folder is writable
tmp/ folder structure, address issue #13
added main .gitignore
functional tests for UserController and ActivityHistoriesController
Merge branch 'status-ap-report'
updated unit and functional tests
Merge branch 'master' of github.com:openwisp/OpenWISP-Geographic-Monitoring
UX notice and alert improvement + gitignore update
user management interface
User and Roles management interface + unit and functional tests
added an access denied response template, unit tests updated
added "logged in as <user>" on top right
added mini profiler to development environment
groups crud + various others improvement listed below:

* Groups model, CRUD, migrations and tests
* Improved user management interface + tests
* added a menu
* added "logged in as <username>" in top right bar
* other UX improvements
toggle monitor + various other UX improvements