Better error handling

This branch

* Raises on all otherwise unhandled errors in 400-599 status range
* Handles new 415 Unsupported Media Type error
* Parses the documentation URL returned by the API
Doc new Error methods
Extract error lookup into methods
Fix indentation

:heart: @jdpace
Clearer docstring for ClientError, ServerError
Merge pull request #308 from octokit/better-error-handling

Better error handling
Pull stray docs into README
Merge branch 'master' of https://github.com/octokit/octokit.rb