Implement action bar, overflow actions and styling

At first wanted to make the currency settings and budget settings as
separate action buttons in the overflow, but that would be too much work
for right now. Decided to leave it as it is, and make them separate in the
future versions. Did some other stuff too along with making the Action
bar.

- Change a theme to Holo light
- Create the action bar overflow menu, that's being show on devices
that have the hardware menu button
- Add the Settings and Help buttons to the action overflow
- Delete the unecessary action bar menus in other activities
- Rename the BudgetSettingsActivity to SettingsActivity as it contains
budget and currency settings
- Make labels in every activity the app name
- Add 4 icons for the action overflow in different resolutions
- Add "back" navigation buttons to AddExpenseActivity,
  ShowExpenseActivity, SettingsActivity.