BUG Incompatibility with 3.1 Object extension syntax

In Silverstripe framework 3.1 use of Object::add_extension is deprecated, and $objectclass::add_extension should be used instead.

Object::has_extension doesn't seem to work at all with the old syntax, although it should still work with a deprecated error (which is a problem on the framework's part, not this module).

I've updated this module by making this module adhere to the new 3.1 syntax.

Please note that fix is NOT compatible with 3.0 and should not be merged into the same branch, otherwise others will end up with broken 3.0 installations. :) Suggest a new branch for 3.1 compatibility?