BREAKING: HTTP client API tweaks

To avoid namespace clashes and to allow easier var referral:
* Rename: `init` -> `init-client`.
* Rename: `get`  -> `http-get`.
* Rename: `post` -> `http-post`.

Other changes:
* API change: `http-get` and `http-post` now take mandatory url, other args destructured.
* API change: args renamed: `:body`        -> `:data`,
                            `:binary?`     -> `:binary-resp?`,
                            `:keywordize?` -> `:keywordize-headers?`.
Update README for new HTTP client API
Rename `transform-header` -> `normalize-headers`