Some additional request keys + custom HTTP clients

[kfyodor]

Hi,

First of all, thanks for the library.

I added some additional API request keys / features I needed for my projects (all changes are backwards-compatible):

• ability to pass request, person, framework and context keys to the Items API
• ability to use a different HTTP Client (for example, someone might want to use persistent connections etc.)
• slightly refactored code responsible for request parameters building, because it became too verbose after adding extra keys.

Hope it's okay!

[marcomorain]

Thanks for the PR @konukhov!

I'm not working on Clojure day-to-day anymore - I've asked @conormcd and @gordonsyme to review this.