Add support for handling keys with a pass-phrase

[pazdera]

Currently, if your repository key is protected by a passphrase, dr will fail using it when signing builds with the repository key.

 What needs to be done

1. Investigate how best could we do this (prompting the user every time, caching the key somewhere, using a some GPG service to do it)?
2. Implement it into the key management class in dr.