Add signing transactions mechnism for Bitcoin

[Midren]

Add signing transactions for Bitcoin. For now, we will be using only P2PKH.

Describe the solution you'd like
Implement a finite state machine to sign transactions similar to the one, that we have for Skycoin.

Additional context
Message for getting address should be written in a files fsm_bitcoin and fsm_bitcoin_impl.

Possible implementation
On the hardware-wallet part, we need only need to get signatures later that will be formed on the computer. In Bitcoin, we only need appropriate keypair and a hash of the previous transaction to get signatures, but it is important to send output addresses and number of bitcoins, so user will see to whom he sends coins.

• Manage getting outputs, so the user can confirm it.
• Sign double hash and send signature using protobuff.
• Write tests for it.

[Midren]

For now, we only need P2PKH, so TransactionSign at the end should return a list of signatures, which later will be used in unlocking script.

Below plan, how communication will be fulfilled:

[Senyoret1]

@Midren I made some tests and I think some changes are needed:

• When signing a transaction, the public keys must be returned too, as currently only the signatures are being returned. This is because the SigScript for unlocking a p2pkh input needs the signature, the sighash code and the publick key. If the Skywallet returns the signature and the public key, the client could simply use 01 as the sighash code. Returning the 3 elements in a string is also an option, but the sighash code should always be 01.

• There is a problem with the decimals. If you try to send a value with more than 6 decimal places, there is an error mentioning an incorrent number of decimals and droplets. That appears to be some code related to Skycoin getting on the way.

• I'm not sure if this is related to the firmware or the daemon, but the input objects sent to the Skywallet for signing a transaction only have 2 properties: index and prev_hash. The problem is that for signing a transaction 3 things are needed: the hash of the transaction in which the utxo was created (the prev_hash property appears to be just that), the index of the utxo in the transaction in which is was created and the index of the addresses which must be used for creating the signature. One of the 2 last elements is missing, and I don't know which one is the one the index property expects.

[Midren]

@Senyoret1

• I think it would a better idea to keep transaction signing doing one job and just add another message, that will return public keys, so for different coins will be the same behavior.
• the index of the utxo in the transaction in which it was created is not needed for skywallet, as it is needed only for forming a transaction