[WIP] Added property helper

Port of [this PropertyUtil class](http://svn.apache.org/viewvc/jackrabbit/trunk/jackrabbit-jcr-tests/src/main/java/org/apache/jackrabbit/test/api/PropertyUtil.java?view=markup) from Jackrabbit.

I find it a bit worrying that this seems to be in some `test` package. I wonder if it is actually used by Jackrabbit.