Maven: Perform GPG signing only when explicitly requested, re #3

Improved fix for GPG signing so I can turn it on explicitly when
releasing to Maven Central, but it stays turned off for the rest of the
time so it doesn't get in the way of everyone else.
Hard-code GPG signing argument into POM to sign on release to Maven Central

Because there is no sane way to pass command-line arguments through to
the "release:perform" process. Christ I hate Maven...
Merge branch 'master' into feature/remove_checked_exceptions
Made XMLBuilder implementation classes final, re #4

Prevent subclassing of these implementations since that would get
weird quickly if someone did so without replacing all of the necessary
pieces, and such a person would likely be better off creating a new
implementation anyway based on BaseXMLBuilder.