BUG Fix regression in IE no-cache https file downloads.

Currently IE6-8 will refuse to download files over HTTPS with default
Framework settings.

Currently the HTTP::add_cache_headers competely overrides Cache-Control
headers on each request, so there is no way to inject custom headers
from the API-consuning methods.

Also of note: adding no-store header also fixes the issue but will
prevent proxies from caching the request body (which they do when using
no-cache). So the setting max-age to some low number is a better choice
here.
Merge pull request #2356 from mateusz/ie-download-fix

BUG Fix regression in IE no-cache https file downloads.
FIX webfonts in preview iframe breaking admin fonts

In IE8, when a page in the preview iframe uses a web font (not just
references it in CSS, but actually has glyphs present in the page),
the fonts in the containing admin interface will become corrupted
(display as all squares) until the mouse moves.

This commit uses the technique described at
http://stackoverflow.com/questions/9809351/ie8-css-font-face-fonts-only-working-for-before-content-on-over-and-sometimes#10557782
to cause the admin panel to re-calculated all it's fonts
after the preview iframe has loaded, fixing the glitch
Merge pull request #2351 from hafriedlander/fix/ie8brokenfonts

FIX webfonts in preview iframe breaking admin fonts
FIX Correct Zend_Locale fallbacks in i18n/DateField/DateTimeField

Due to the recent change of translations to transifex, some
locales changed their names, which prompted a fix to
i18n::get_available_translations() (see 00ffe7294).
This caused a regression where short locales are determined
from the YAML file names (e.g. "en"), but weren't matched up
with fully qualified locales from get_available_translations() (e.g. "en_US").
Since this list is used in the admin/myprofile dropdown for the Member.Locale value,
it didn't match up with any entries and defaulted to the first one ("Africaans").

Note that the behaviour of admin/myprofile is still a bit weird:
It defaults the locale on new members to the one set for the current administrator.
So if a site defaults to en_US in _config.php, but the admin happens to view
his backend in de_DE, all members he creates default to de_DE as well.

Thanks to @tractorcow for contributing and peer reviewing!
Merge branch '3.1.0' of https://github.com/silverstripe/silverstripe-framework

Conflicts:
	i18n/i18n.php