Also updated Makefile to include these tests in the "make test" suite.
It became awkward to automatically detect the .py test files (they don't
really follow any particular naming convention) so they're listed
explicitly. To make things easier to follow I also listed out the other
stdlib test files explicitly. Long term I think we should be listing all
files this way (including .go sources) so that things are less magical.

* ToInt() function that dispatches to `__int__` slot
* ToIntValue() that does the above and then tries to fit it into a plain `int`
* Switch some functions that were doing IntType.Call() to these functions which fixed some unsafe conversions

pip install --upgrade flake8  # http://flake8.pycqa.org
python2 -m flake8 . --exclude=third_party --ignore=E1,E306 --max-line-length=80

* move functools.py to stdlib
* move md5.py to stdlib
*  move re.py re_tests.py sre_*.py to stdlib

Fixes: https://github.com/google/grumpy/issues/123

* Remove dependence on pip.
* Add osx to Travis test matrix.
* Remove pip upgrade and sudo requirement for Travis config.

* added groupby, izip_longest, permutations, product, combinations and combinations_with_replacement.
* added corresponding tests.
* make chain to be a iterator
Signed-off-by: Cholerae Hu <choleraehyq@gmail.com>