Fix selection of dependent tuples in dj.BaseRelvar.del()

* When deleting tuples from a parent table, all affected subtables
  from dependentent tables also have to be deleted.
* The dependency relationship is based on the primary key fields
  of the parent table.
* When restricting dependent RelVars in del(), the restricting
  RelVar has to be projected onto its set of primary key fields.
* Otherwise, some dependent tuples will be missed and the del()
  operation will fail at the level of MySQL
Merge pull request #25 from ahoenselaar/future

Fix selection of dependent tuples in dj.BaseRelvar.del()