Fix min/max functions with generators, and 'None' default

[gazpachoking]

I allowed a few fixes to creep into this PR, I can separate them out if desired. This PR addresses:

• There is an issue with the min/max functions consuming the first argument of iterators that cannot be looped over more than once. max() over itertools.ifilter() is broken in 0.18.0 #510
• The new min/max functions did not allow None as a default.
• Aligned the error message when an empty sequence was passed to the new min/max functions to be the same as the builtin min/max error message.

[gazpachoking]

Oops, didn't see we already had some tests in test_builtins. I'll just add the appropriate new tests there.

[gazpachoking]

A couple more changes crept into this PR, updated the original PR text to reflect this.

[gazpachoking]

Not sure why travis isn't adding the results here, pretty sure the 2.6 failures have nothing to do with this PR though. https://travis-ci.org/PythonCharmers/python-future/builds/599676065

[megies]

I can confirm this fixes an issue for me which looks like what is described in #510 first post, see #510 (comment).

I recommend merging and releasing this