Signing support (improved)

Implementation of issue #93.

This is based on the same code as #103 but branched off 192ff8b0a2489081663a1d1ca55e16aae7b24bc8 (which was HEAD when I forked).

It also includes my own changes that allows for keyring, key and passphrase configuration as properties in maven's settings.xml instead of in the POM, with support for prefixes to use different keys for different projects. The keyring, key and passphrase can be set in the POM if desired.

I've also modified it to be a bit more verbose about what's going on and to warn instead of throwing an error if the keyring, key and passphrase properties aren't set to allow building the project (and not signing the .deb) without setting up a keyring.