Use SelectInfo class in Django 1.6+

This is a new data structure that includes the column and the field
instance.

Signed-off-by: Byron Ruth <b@devel.io>
Get join fields from `get_path_info` on the m2m related field

This is has been added in Django 1.6.

Signed-off-by: Byron Ruth <b@devel.io>
Fix initial connection use a None for join columns

Inspecting the source reveals this is the proper way to represent the
first left-hand side of a join sequence. Otherwise the connection will
be treated as a separate connection and will be aliased.

Signed-off-by: Byron Ruth <b@devel.io>
Fix LazyModelTrees test to prevent testing external state

Signed-off-by: Byron Ruth <b@devel.io>