BG-486 / issue #37: Adding an informative exception when trying to post abstract classes.
Merge branch 'development' of https://github.com/okb/Pomona into development
Altering PostAbstract test to reflect canges in error handling
Adding an extra test that posts an abstract class, which fails on the server
Merge pull request #49 from doffen/development

Support for isAbstract property, and two tests testing that it's handled correctly