Ricecooker

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* Added support for non-mp4 video files, with auto-conversion using ffmpeg. See ``git diff b1d15fa 87f2528``
* Added CSV exercises workflow support to ``LineCook`` chef class
* Added --nomonitor CLI argument to disable sushibar functionality
* Defined new ENV variables:
* PHANTOMJS_PATH: set this to a phantomjs binary (instead of assuming one in node_modules)
* STUDIO_URL (alias CONTENTWORKSHOP_URL): set to URL of Kolibri Studio server where to upload files
* Various fixes to support sushi chefs
* Removed ``minimize_html_css_js`` utility function from ``ricecooker/utils/html.py``
to remove dependency on ``css_html_js_minify`` and support Py3.4 fully.

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* Changed default logging level to --verbose
* Added support for cronjobs scripts via `--cmdsock` (see docs/daemonization.md)
* Added tools for creating HTML5Zip files in utils/html_writer.py
* Added utility for downloading HTML with optional js support in utils/downloader.py
* Added utils/path_builder.py and utils/data_writer.py for creating souschef archives
(zip archive that contains files in a folder hierarchy + Channel.csv + Content.csv)

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* Sibling content nodes are now required to have unique source_id
* The field `copyright_holder` is required for all licenses other than public domain

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* Added `JsonTreeChef` class for creating channels from ricecooker json trees
* Added `LineCook` chef class to support souschef-based channel workflows

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* Added `language` attribute for `ContentNode` (string key in internal repr. defined in le-utils)
* Made `language` a required attribute for ChannelNode
* Enabled sushibar.learningequality.org progress monitoring by default
Set SUSHIBAR_URL env. var to control where progress is reported (e.g. http://localhost:8001)
* Updated le-utils and pressurecooker dependencies to latest

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* Clarify ricecooker is Python3 only (for now)
* Use https:// and wss:// for SuhiBar reporting