feat: adds rack middleware to make request verification easier in rack apps.

[philnash]

This adds a class that can be used to verify requests with a public key in rack applications like Rails or Sinatra. This was based on my work on the twilio-ruby rack middleware here.

This also changed the class to fail verification if an error was thrown. In the case where a signature was missing, OpenSSL would throw an error instead of returning false for a invalid signature. I think it's more appropriate for the verification to fail than to throw an error at this point, especially since the error came from OpenSSL and wasn't otherwise clear.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the master branch
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

[philnash]

This probably wants another look after #427 is merged just to make sure.