* 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>

I sense that you folks are making progress on `re`, etc.  The next line points readers to the wiki list that should be more complete and easier to keep up to date.

When comparing two objects and the appropriate rich comparison
method doesn't exist, the `__cmp__` method should be tried. For
example, if `a == b` is executed and neither `a` or `b` have
the appropriate rich comparison methods, then the equivalent
of `a.__cmp__(b) == 0` should be tried.

Lots of tests for both 3-way comparison and rich comparison were
added.  The comparison implementation in the runtime should be
fully covered now.

Python gaurantees that its builtin sort for mutable sequences is stable.
For more details, see note (9) in:

  * https://docs.python.org/2/library/stdtypes.html#mutable-sequence-types

I couldn't come up with a decent way to test this.