Adding max age property to Kerplapp repo index metadata.

In order to help address the "freeze attack" identified in the Fdroid
audit support has been added to the fdroid-client & server for a 'max
age' property
(https://gitorious.org/f-droid/fdroidclient/commit/d21788569fe9e38f7ac6c4f2e1a2a42bbc3daa0e).
Presently the fdroid client does not enforce this max age, but we should
add it to our repo index for future support.

A preference screen entry was added to allow manipulation of the max age
timeout. It has been set to the same default value as the official
FDroid repo's maxage property (14 days).

Refs Task #2667