SPIN-1305_custom_date_range_queries

SPIN1305: Add custom date range queries (#7)

* add route for cost-explorer date range for GET, expand key for cache to enable multiple date range objects to be cached.

* add comment to add date range unit test, updated README, etc.

* updates, added input cleansing, and good API errors when input is bad

* added endpoint querystring for StartTime, EndTime to readme

* changes per PR review

* cleaned up logic per PR

* cleaned up debug statements

* clean up time to string conversion

* refactored new code into smaller functions, built unit tests for new code

* sort out unwrapping time errors back to client caller, adding tests for dates

* updated with PR suggestions

* nit

* moar nits