Jmbo-downloads

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. Add `DOWNLOAD_INTERNAL_REDIRECT_HEADER` setting (default 'X-Accel-Redirect') in case a webserver other than Nginx is used.
. Add `DOWNLOAD_SERVE_FROM` setting (default 'LOCAL') to specify whether to serve the files locally or redirect to a remote location.

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. Fix bug where X-Accel-Redirect uses file name override instead of actual file name to serve download, causing a 404 if the file name and override don't match.

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. South dependency on jmbo upped to 0002 migration.

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. Improve templates to better render download categories.
. Override Jmbo's PermittedManager to exclude invisible downloads from querysets.
. Make ImageMod more generic and rename it to TemporaryDownloadAbstract.
. Send signal when a download is requested, allowing other apps to track downloads.
. Miscellaneous small fixes.

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. Add everything to manifest for PyPI release.

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. Initial release.