No immutability

I just noticed one issue with dissoc on dictionaries.

diff --git a/toolz/tests/test_dicttoolz.py b/toolz/tests/test_dicttoolz.py
index 9134276..9b9a735 100644
--- a/toolz/tests/test_dicttoolz.py
+++ b/toolz/tests/test_dicttoolz.py
@@ -103,6 +103,10 @@ class TestDict(object):
         assert d is oldd
         assert d2 is not oldd

+        d = D({'x': {'a': 1}, 'y': 1})
+        d2 = dissoc(d, 'y')
+        assert id(d['x']) != id(d2['x'])
+
     def test_update_in(self):
         D, kw = self.D, self.kw
         assert update_in(D({"a": 0}), ["a"], inc, **kw) == D({"a": 1})

I just added an example test to see that it fails, in short if you have nested dictionaries copy.copy doesn't create completely new and independent objects, but subobjects are shared.
Using copy.deepcopy instead would do the trick (but it's slower obviously)

Start with toolz/tests/test_dicttoolz.py and run the added dissoc case to reproduce the nested-dictionary aliasing. Then trace dissoc and its copying behavior; done means the test passes with nested values independent between the original and returned dictionaries, while accounting for the stated performance tradeoff.