Don't fail when adding records with no parent.

When you use SwatDBRecordsetWrapper::attachSubRecordset() and the
recordset being added contains records that point to records not in this
recordset, those records should be ignored.

For example:

set of images -> 1, 2, 3, 4, 5, 6
set of dimensions referring to images -> (1,2), (2,2), (3,1), (4,10)

If the dimensions are attached to the images the dimension binding
(4,10) which does not match any image should be ignored rather than
causing an error.
Merge pull request #6 from gauthierm/attach-recordset-ignore

Don't fail when adding records with no parent.