Ref #31: Initial support for Hybrid transport (iOS only, non persiste…

…nt) (#51)

An initial rough implementation of hybrid transport - formerly known as Cloud Assisted Bluetooth Low Energy, or caBLE.

## Includes

* Generates QR code for QR-initiated transactions
* Adds WebAuthn example which prints QR code to terminal (`examples/webauthn_cable.rs`)
* Adds submodule fork of `snow` (noise protocol framework) with NIST P-256 curve implementation, as required by CTAP spec (see mcginty/snow#185)

## Limitations and follow ups

* Unable to discover Android devices (created issue #52)
* Does not yet support state-assisted transactions (part of #31)
* Clean up (part of #31)

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "ctap-types"]
 	path = ctap-types
 	url = https://github.com/AlfioEmanueleFresta/ctap-types.git
+[submodule "snow"]
+	path = snow
+	url = https://github.com/AlfioEmanueleFresta/snow.git