Fixed relation handling. Fixed #473, #518, #262.

Fixed bug: crossTable with at least one FK with multiple local columns
Fixed bug: crossTable with more than two FKs
Fixed bug: crossTable with two PK-FK and one PK-nonFK
Fixed bug: crossTable: setRelations(Collection),
removeRelation($rel), save() doesn’t remove $rel.
Added Partial-checking also to CrossFK getter.
Fixed bug: wrong "if ($obj->isNew())” check in getRelation() methods.
Improved performance of hashCode.
Improved ObjectCollection checks
Modified clearAllReferences as in PHP 5.4 there's no workaround like
that necessary anymore.
Fixed internal object relation reference tracking, especially
unlinking back-references.

Added also the table rename fix for #477, so we do not rename tables
anymore. This was necessary because otherwise with this new incoming
tests for relations the test suite won't be green.
Merge pull request #476 from marcj/master

Bugfix for #473.