Add Ledger Nano support

[abourget]

Integration is possible, might require building with CGO_ENABLED=1, which might make cross-compilation a bit harder, we could have a ledger-enabled build for different platforms.

Refs:

• https://github.com/ethanfrey/ledger
  • which uses https://github.com/ethanfrey/hid/blob/master/hid_darwin.go (CGO stuff for all platforms)
• fairy-wallet uses https://github.com/tarassh/fairy-wallet/blob/master/app/actions/ledger.js which points to https://github.com/LedgerHQ/ledgerjs (see docs here: http://ledgerhq.github.io/ledgerjs/docs/ )
• other HID implementation: https://github.com/karalabe/hid

[apimdavid]

+1