[RFC] warn if the system time is wrong

*do not merge yet*

addresses kennethreitz/requests#2170

The naming can be improved. We could probably add better support for windows (I think there the time begins at 1900, should be tested)
How about something like "System time is before 1971, which is way off. This is likely to lead to SSL verification errors."

Any reason to just check 1971? Could hard-code 2014 or something for funsies. :P