Add MainActivity and SettingsActivity

 * MainActivity:
   - It is composed of a Navigation Drawer from which you can access
     the most important activities of the application.
   - The Navigation Drawer opens, either by sliding your finger from
     right to left, or by clicking on the icon-name in the action bar.

 * SettingsActivity:
   - In the future, it will contain all the options for the user. For
     now, it is nothing more than a blank activity.

Signed-off-by: AlvaroOrduna <alvaro.orduna.92@gmail.com>