[WIP] Added CloudFront to list of available CDNs

[phansys]

Added CloudFront to list of available CDNs
Any suggestion or improvement proposal are welcome.

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	yes
Deprecations?	no
Tests pass?	yes
Doc PR	none

TODO:

• Unit testing
• Update model in order to persist flush identifier
• Create command to update flush status
• Update translation catalogs

[phansys]

I have too some ideas about CDN usage / integration. IE, consider the following snippet attached to a lifecycle event or something similar:

#...
    $flushPaths = array();
    foreach ($provider->getFormats() as $format => $settings) {
        if ('admin' === $format || substr($format, 0, strlen($media->getContext())) == $media->getContext()) {
            $flushPaths[] = $provider->getFilesystem()->get($provider->generatePrivateUrl($media, $format))->getKey();
        }
    }
    if (!empty($flushPaths)) {
        $cdnFlushIdentifier = $provider->getCdn()->flushPaths($flushPaths);
        $media->setCdnFlushIdentifier($cdnFlushIdentifier);
        $media->setCdnIsFlushable(true);
        $media->setCdnStatus(CDNInterface::STATUS_TO_FLUSH);
    }

This also should take care about new_on_update form setting and only flush the CDN paths when it is set as false, because the "magically generated" new medias are fresh and aren't cached yet while the previous ones has kept immutables.

What do you think about?

By other side: should "cdnIsFlushable" become "flushingCdn" (deprecating the previous one)? since it is not being used to indicate whether the CDN supports flush, but to indicate that the flush is in process.

[phansys]

I've rebased PR, squashed commits and added this logic in BaseProvider::transform() method.
Any concern about this feature will be great.

/cc @rande.