WIP HMAC authentication view
Update the base ApplicationException with custom headers

Also update the exception_handler to set those headers on errors.
Updated HMAC with working implementation
Tests for HMAC authentication / failure states
Added docs and cemented the public API for HMACView
Index the authorization header as it should always be included
Added docs for authentication with HMAC signed requests
Link in the table of contents
Reworded HMAC definition
Merge pull request #39 from ox-it/authentication-hmac

HMAC authentication view