minimal support for external extensions to pgjdbc datatypes

[frode-carlsen]

Since #178 is on hold, I'd like to make the following minor changes to the existing project to ease creating external datatype extensions to the library.

[kdubb]

Does this conform to our checkstyle? I thought "protected" should be on the same line as the method definition.

[frode-carlsen]

interesting, it passed the checkstyle tests so looks like that's not enforced.
I used automated refactoring to make the change, so I have no qualms about changing it to be on the same line. Will check in

[jesperpedersen]

@frode-carlsen Could you fix the indentation of the pom.xml file change ? Then rebase, and squash. Thanks

[frode-carlsen]

ok, made some cosmetic changes to make it align same as plugins with above/below, but the whole pom.xml needs reformatting. It's a mix of tabs and spaces and improperly aligned child elements. Can do it here, but think perhaps it should be a different PR?

[jesperpedersen]

@frode-carlsen Just format to spaces here, but squash your commits too

[frode-carlsen]

how about this?

[jesperpedersen]

Rebased, and merged. Thanks for your contribution !