[dev] heads up: API change in Rdo queries

[Chuck Hagenbuch]

I'm going to need to change how Rdo handles relationships and joins a  
bit. The relationships arrays will hopefully remain unchanged, though  
with a few more options. While I'm at it I'm going to rename the  
addRelationship() method on Horde_Rdo_Query to addJoin, and bring it  
closer to SQL as we've done with some of the rest of Rdo.

-chuck