Requests-cache

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* Add gridfs support, thanks to chengguangnan
* Add dynamodb support, thanks to ar90n
* Add response filter 104, thanks to christopher-dG
* Fix bulk_commit 78
* Fix remove_expired_responses missed in __init__.py 93
* Fix deprecation warnings 122, thanks to mbarkhau
* Drop support for python 2.6

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* Support PyMongo3, thanks to craigls 72
* Fix streaming releate issue 68

* Fix ability to pass backend instance in `install_cache` 61

* `ignore_parameters` feature, thanks to themiurgo and YetAnotherNerd (52, 55)
* More informative message for missing backend dependencies, thanks to Garrett-R (60)

* Better transactional handling in sqlite 50, thanks to rgant
* Compatibility with streaming in requests >= 2.6.x

* `expire_after` now also accepts `timedelta`, thanks to femtotrader
* Added Ability to include headers to cache key (`include_get_headers` option)
* Added string representation for `CachedSession`