Created the settings activity (to put Dropbox Sync configurations), and
the button Dropbox Sync on the main menu of the app.

Also renamed some files and removed some unused code.
Removed listener of 'Nova Materia' button from Anonymous class and added it WelcomeActivity, which now also implements OnClickListener
Added Dropbox AccoutManager, and .gitignored the keys
Changed some stuff required by Dropbox API. It's working. Now it's needed
the code to modify whatever files we want.