The documentation about the __future__ features [1] only lists the
features available for that particular version, without specifying when
they were added and their effects.
It would be nice to have a table with |feature|version added|effect|.
A better place for this table could also be the page of the __future__
module [2].

[1]: http://docs.python.org/reference/simple_stmts.html#future
[2]: http://docs.python.org/library/__future__.html

I composed a list of __future__ features and linked the respective PEPs.

Even though the language reference would be a better place to store such
general information (being PEP'd and all) I found the library
reference's __future__.py documentation to be a cleaner host for that table.

Python 3 docs would need to add the barry_as_FLUFL feature to that table.

For Python3, the table should start empty as of 3.0/3.1

Accepted, except for two things:

* I'd drop the "a0" suffix for the mandatory version. "2.2" instead of
"2.2.0a0" is much better to comprehend.

* Why the ugly markup with the PEP refs? ``:pep:`number`: blah`` should
work well.

For 3k, all previous future features are still supported, so they can go
in there as well.

Implemented proposed changes.

Additionally, I'd change line 13 to state either "future statements" or
"`future`:ref:" instead of "future_statements", which does not make
sense in normal, unmarked text.

I added a reference in line 13, changed the table header not to use
colspans (the latex writer doesn't support that), and committed as r74791.