[WIP] Automatically use binary frameworks from GitHub Releases

Resolves #6.

**To do:**

 - [x] Automatically fetch releases for GitHub repository dependencies
 - [ ] Automatically paginate through all releases
 - [x] Download release assets with content type `application/zip`
 - [x] Unzip downloads into a temporary folder
 - [x] Drop binary frameworks into `Carthage.build/` based on platform
 - [x] Terminate asset/release fetching early when a matching binary is found
 - [x] Cache assets in `~/Library/Caches`
 - [ ] Reuse existing downloaded assets
 - [x] Skip repository checkout when binaries are found
 - [ ] Remove old repository checkout when binaries are found
 - [ ] Only load Git credentials once
 - [ ] Add documentation
 - [x] Add logging
 - [ ] Split up signal chain
 - [ ] Only download binaries when updating or bootstrapping with `--build`, or checking out with `--use-binaries`
 - [ ] Test unzipping
 - [ ] Test framework extraction from a zip
Ah, indeed.