Requests-cache

* `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`

* Fix bug in reading cached streaming response

* Fix compatibility with Requests > 2.4.1 (json arg, response history)

* Monkey patch now uses class instead lambda (compatibility with rauth)
* Normalize (sort) parameters passed as builtin dict

* Requests==2.3.0 compatibility, thanks to gwillem

* Check for backend availability in install_cache(), not at the first request
* Default storage fallbacks to memory if `sqlite` is not available