Let's do some monetizing

GB will provide some premium features to the users that supported
development by means of a donation. Since there's no support for
distributing paid "unlock keys" via play store in Slovakia I decided
to make an interface against PayPal services.

External Web Service has been implemented that utilizes PayPal API to
check the status of transaction IDs. GravityBox calls this web service
to verify transaction ID entered by user. If transaction ID is valid,
premium features are unlocked. The advantage of this solution is that
users that already donated can use their existing (old) transaction IDs
to unlock features. This will avoid discriminating users that already
supported development and deserve premium features right from the start.

The initial list of "premium" features is rather funny, but let's start
with something  less aggressive first not to depress people too much :)
These are: backup and restore of GB settings.

Since we are opensource, it makes the monetizing system too vulnerable.
Skilled user could rather easily hack it but well... Better than
nothing. Web service part coded in PHP stays closed-source for obvious
reasons.