Verify X-GitHub-Event Header

[bradshjg]

Fixes #24 (well, should hopefully make it much more clear to users what the error is).

• Previously, this would throw a KeyError if the header was missing. This thrown error would get caught and returned to the caller as a 500 error without any context. It would also leave a potentially confusing traceback in the application logs.
• Now, we first check if the header is present and if it's missing we alert the caller via a 400 response with appropriate description as well as leaving a much more relevant error log event.

[bradshjg]

@Napo2k No pressure to review this, but any review certainly appreciated 😄

At some point I'd love to refactor this library, but this implementation is kinda trying to follow in the spirit of the existing code (for better or worse).