Recreated signing from mpoindexter's fork.

Added the changes from
https://github.com/mpoindexter/jdeb/commit/3254f58 manually since a lot
of code has changed since then.
Added support for reading signing properties from settings.

Added support for reading signing properties (keyring, key and
passphrase) from settings, allowing storage of sensitive information
outside the pom.

A configurable prefix can be used 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.
Warning instead of failing when missing signing properties.

Issuing a warning instead of an exception when signing properties are
not set means that the build won't fail if you build on a system that
doesn't have the signing keys available. This allows for setups where
packages are generated and signed by an automated build system while
still allowing developers to compile the code without the need to set up
a keyring.
Throwing a proper exception if a non-existent key was specified.

Previously, a NullPointerException was thrown if the specified key
didn't exist in the keyring which could be very confusing. Now a proper
exception with a proper error message is thrown instead.
Added a log message stating that we're signing the .deb
Added documentation for signing packages
Fixed some formatting on the documentation
Removed some elusive tabstops from the documentation
Modified the version so we can use it internally