Add BTChip wallet plugin

Hi 

Here's a plugin to support BTChip hardware wallet - based on Trezor plugin. Transaction and messages signing are supported, message encryption is not supported.

Things to review : 

- is_available condition (seems broken in master)
- BIP 44 path harcoded in address_id and derive_xkeys

Let me know if you want some samples before the commercial launch or if you can upgrade an existing sample (for Thomas mostly, I guess)

Cheers

Nicolas