Pathgather

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* Allow filtering of paths and users by custom query
* Added `get_by_email` to users class

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* Use arrow dates on formatted UserPath

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* Add support for filtering content
* Add support for updating a piece of content

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* Add pathgather.skills API for gathering, listing and adding skills
* Add support to invite a user from a gathering
* Remove `add_user` feature, since the API doesn't work
* Add ability to list content in a gathering and delete content
* Add support to list paths within a gathering
* Update production path and user models

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* Fetch content completions and starts for users
* Register content completion for a user and a piece of content
* Fetch path starts and completions for users
* Gatherings API for listing, getting, creating, updating and deleting Gatherings
* List users within a gathering
* Add or remove users from a gathering

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* Updated content models with new fields
* Added support to specific content provider by name or custom_id
* Fixed doc issue with content.create
* Added pathgather.models package to setup.py
* Added SSL check disable flag on client and HTTP proxy settings