migrate to API-SECRET header

@jasoncalabrese pointed out that some agents do odd things with
underscores and that using a hyphen is more common.
This also adds tests to ensure that the API_SECRET handling works as
expected.
Revert "migrate to API-SECRET header"

Cherry-picked in the wrong order

This reverts commit 8e56ef17529f3867e0d907a81f95e40276814037.
Merge pull request #51 from nightscout/rebase/settings

Add Settings
Merge pull request #35 from nightscout/feature/base-api

Base API - settings, basic auth, and experimental endpoints