Issue 15: JOIN Support

The two tests as mentioned in #15 are passing now.
Added some more tests to phpcr-api-tests (but those haven't been merged yet) for LEFT/RIGHT joins.

For now, only DescendantNodeJoinCondition and EquiJoinCondition are implemented. 
ChildNodeJoinCondition and SameNodeJoinCondition are not implemented yet.

It does contain a BC break, but I don't know how much impact it has.
The default alias was 'n', that is now 'n0'. So this might cause problems somewhere...

And I haven't been able to test this with a real project yet, so we maybe miss some issues that aren't covered by tests yet.