(#19) add "Manage Users" that allows admins to see user list and change roles for them

* add "Manage Users" page, angular controller, route, service, template
* add `currentCompanyStore` and `userStore` services
* add `index`, `update` and `roles` actions to `api/users`
* add `current` action to `api/companies`
* fix associations
* move "Manage Projects" button to navbar
* add services tests (jasmine)
* add controller tests (rspec)