Cumulative patch with refactored Raniz's changes

Support for package signing for maven plugin, including:

* Automatic lookup of secure ring location (keyring) if not set.

* Support for reading signing properties (keyring, key and
  passphrase) from settings, allowing storage of sensitive information
  outside the pom.

* Configurable prefix to define different keys for different packages,
  the default prefix is "jdeb." resulting in the properties
  jdeb.keyring, jdeb.key and jdeb.passphrase being read from the
  properties in the active profiles.

* Log message stating that we're signing the `.deb`.

* Documentation for signing packages.

The code is mostly the one written by Raniz, but it renewed and reformatted a little bit to better conform surrounding code style. I've tested the result on a simple module and I was able to build and sign my package using both `pom.xml` and `settings.xml` configurations on my Linux machine (unfortunately, I don't have a windows installation to test the thing). I've also renamed maven parameter `signParameterPrefix` -> `signCfgPrefix` to keep parameter names short. I've also added some warning suppression annotations to make my IDE happier.