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>