Apply request and response middlewares
Move creation of sawyer request out of client
Use `Client.Header` instead of a middleware system

* Remove introduced middleware system
* Expose `Client.Header` instead for globally applied headers

A caveat is that Go doesn't apply `Host` on the header but consulting
`Request.Header`. Details see https://code.google.com/p/go/issues/detail?id=7682.
Add more test case to setting header
Merge pull request #60 from octokit/middleware

Apply global headers