Trezor plugin fixes

I ran into a couple of issues when trying to create a transaction with the Trezor. This pull request fixes both of them.

1) Wallet.get_address_index() now returns the purpose and coin type, so it shouldn't be hard coded in TrezorWallet.address_id()
2) TrezorWallet.tx_inputs was expecting each input to have an 'address' key, which is only the case for non-coinbase transactions