Timed clipboard clearing

[t-fe]

For security reasons KeePit should clear the clipboard automatically some time after you've copied one of your passwords into it.

[DannyGB]

This is a feature that was intended from the beginning, and the code actually exists in the app, unfortunately there is a bug in the framework that crashes the app consistently so a work around is required.

See #2

https://bugs.launchpad.net/ubuntu/+source/ubuntu-ui-toolkit/+bug/1457361