Add fixer for itertools

[ucodery]

Use of ifilter, imap, and izip as generator forms of PY2 builtins are replaced
with future.builtins versions that are always generators

This fixes issue #424

[jmadler]

Thanks for your help! :)

[ucodery]

I am not sure why issue #424 was opened in the first place. This worked fine before this merge.

from itertools import imap

print imap(pow, range(1, 10, 3), range(1,4))
(python-future) MBP-jpaige% python -m futurize ./test.py
RefactoringTool: Skipping optional fixer: idioms
RefactoringTool: Skipping optional fixer: ws_comma
RefactoringTool: Refactored ./test.py
--- ./test.py	(original)
+++ ./test.py	(refactored)
@@ -1,3 +1,6 @@
-from itertools import imap
+from __future__ import print_function
+from builtins import map
+from builtins import range

-print imap(pow, range(1, 10, 3), range(1,4))
+
+print(list(map(pow, list(range(1, 10, 3)), list(range(1,4)))))
RefactoringTool: Files that need to be modified:
RefactoringTool: ./test.py

Sorry about this, but I think this change should be backed out. The same logic was already implemented in src/future/moves/itertools.py