Support Hash mode in AccessToken#initialize

[pboling]

In the README I've documented use of a Hash as well as a callable for a HTTP verb-dependent AccessToken.

@mrj added support for the callable.

I didn't add support for the Hash mode yet. 😬 Got a little ahead of myself.

- a Hash with verb symbols as keys, for example: {get: :query, post: :header, delete: :header}.

• Support for Hash mode
• Hash mode also needs to be described in the AccessToken#initialize docs

Thanks to @mrj for calling it out!

[pboling]

Implemented in #683