Update request signing to work for any content type

[shaydewael]

Summary

Fixes #41

Server is currently looking in request.data for raw request body in the verify_signature method. This is fine for intended functionality (for use with the Events API), but developers that want to use the function to handle other content-types (like with Slash commands) can't right now. This fixes that by using request.get_data() instead, which works regardless of content-type 😄

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.

[wooddar]

🎉