Major changes:
* Move to Rust 2018 edition #125
* Remove dependency on boxfnonce #121
* Reworked error handling #130
* Added a higher-level AuthenticatorService that can use multiple backends #123
* Changed the C API to use the new AuthenticatorService #123
* Added a Status channel for backends #122
* Now obtaining HID report sizes from the descriptors #112
* Add authenticator USB and Firmware details to the C API #93