Link to parent causes reference cycles

[bmerry]

With addict 2.0.0, this code generates a reference cycle and may lead to memory leaks:

from addict import *
a = Dict()
a.x.y = 1

Here a contains a reference to a.x, and a.x.__parent__ points back to a, thus creating a cycle.

__setitem__ should probably clear __parent and __key after they've served their purpose.

[mewwts]

Thanks @bmerry. I've release 2.0.1 with the most of the fixes you've pointed out. I want to encourage you do a review of #84, and submit a PR in the future. You've already helped make addict a lot better.