see description

- feature: an ability to change base location;
- fixed bug: "if synchronization is interrupted then after that if user
trying to leave screen with map which opens SynchronizationActivity,
then instead of been killed and returns to previous activity the screen
with map recreates" (may be not fixed, but I doesn't see it again since
trying to fix it until now);
- refactoring (removed Keys class, etc.);
- displaying text label if offer list is empty;