Ship Akavache.Http

This is the top-level PR for shipping Akavache.Http, a new library that helps make HTTP requests on mobile (and desktop!) applications efficient.

## [Check out the elevator pitch / notes here](https://github.com/github/Akavache/blob/a4dec239f6bbefa60a7123caa143377f33782681/Akavache.Http/README.md)

## TODO

- [x] Write README
- [ ] Everything Else