Download setup.ini only if server copy is newer

[maphew]

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[maphew]

Although implemented specifically for saving downloads of setup.ini, it's generalized so also applies to package downloads. Previously we just relied on hash md5 changes for deciding on whether to download or not. (Ideally, we should also be relying on hash check of setup.ini. I don't think that's implemented upstream either though(?) If not, that should be done too.)

The new functions:

 + url_time_to_dateime: Convert "last-modified" web server header to a python datetime object
 + datetime_to_unixtime: Convert a datetime object to unix UTC time

... are not robust, and likely susceptible to differences in locale syntax. I [asked SO for an idiomatic approach][0], but no solutions yet. I've spent about 8 hours on these few lines; date and time is hard! I'll be very surprised if there no bugs in this change set.

[0] http://stackoverflow.com/questions/29314287/python-requests-download-only-if-newer

[maphew]

Useful threads:

• http://stackoverflow.com/questions/11405049/how-to-get-the-file-modification-date-in-utc-from-python
• http://stackoverflow.com/questions/24182623/python-converting-datetime-to-be-used-in-os-utime
• http://stackoverflow.com/questions/8777753/converting-datetime-date-to-utc-timestamp-in-python/ (very good)