Webauthn support

[Firehed]

Closes #2, closes #1

For the most part, this is just a tremendous amount of format shifting, and some updated demo code which should get migrated to Firehed/u2f-php-examples. Thanks to the new interfaces introduced in 1.1.0, there should be no breaking changes.

Right now, this needs the following changes:

• Move CBOR decoder into separate repo (unfortunately I found some minor compatibility issues with the existing ones) and add it as a dependency
• Rename the additional WebAuthn-specific data structures
• Encapsulate the "demo" parsing logic of client requests (the POSTed JSON structure will need to be documented)
• Examine the adjustments in the Server code to ensure no weird cross-origin/Relying Party issues are exposed
• Resolve the issue where WebAuthn expects a single challenge across all registered keys, where the old format did not
• Test in Chrome
• Move (or consolidate) the demo code
• Look at tidying up classes/interfaces for reused objects (will probably close Interfaces for Registration and Request (and SignRequest…)? #4)
• Update the README, mark as breaking change with 2.0 (and mbstring overload warning)

Also, it would be worth considering other formats and verification procedures, though that's out of scope for now (basically the entirety of WebAuthn section 8)

[codecov]

Codecov Report

Merging #11 (4edfcb3) into master (58a97aa) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##              master       #11    +/-   ##
============================================
  Coverage     100.00%   100.00%            
- Complexity       103       134    +31     
============================================
  Files             18        21     +3     
  Lines            360       545   +185     
============================================
+ Hits             360       545   +185     

Impacted Files	Coverage Δ
src/Server.php	100.00% <100.00%> (ø)
src/WebAuthn/AuthenticatorData.php	100.00% <100.00%> (ø)
src/WebAuthn/LoginResponse.php	100.00% <100.00%> (ø)
src/WebAuthn/RegistrationResponse.php	100.00% <100.00%> (ø)

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