Add support for CAP0015 (FeeBump).

[abuiles]

What

Add support for CAP0015.

Why

Protocol 13 introduces a new kind of transaction which allows other accounts to pay the fee for a given transaction.

This pull request adds a new helper functions which makes it easy for consumers of this library to create a fee bump transaction.

This PR:

• Add .buildFeeBumpTransaction to TransactionBuilder which makes it easy to create FeeBumpTransaction.
• Adds a new helper function to the Transaction class which allows consumer to know if the transaction is a FeeBump transaction or not.
• Adds a feature flag which allow consumers of this library to create V1 (protocol 13) transactions using the TransactionBuilder.
• Extends TS definitions.

Fixes #319

How to create FeeBump transactions?

Assuming you have a valid version of stellar-core and Horizon running with protocol 13, you can use the following example to create a fee bump transaction (replace keypairs):

let sourceKP = Keypair.fromSecret("SAQZZ7K4YUOEX3MMLX5YM4VCK4E7NSKRODKNPNYCMH44JGGXNLUDNZH7");
// account to which the inner transactions belongs to
let account = await server.loadAccount(sourceKP.publicKey())
let PASS_PHRASE = "Standalone Network ; February 2017";
let destination = 'GDQERENWDDSQZS7R7WKHZI3BSOYMV3FSWR7TFUYFTKQ447PIX6NREOJM';
let amount = '20';
let innerTX = new TransactionBuilder(account, {
  fee: 100,
  networkPassphrase: PASS_PHRASE,
  timebounds: {
    minTime: 0,
    maxTime: 0,
  },
  v1: true,
})
    .addOperation(
      Operation.payment({ 
        destination,
        asset: Asset.native(),
        amount
      })
    )
    .addMemo(Memo.text('Happy birthday!'))
    .build()
innerTX.sign(sourceKP);

// Keypair for account paying for the FeeBumpTX
let feeSourceKP = Keypair.fromSecret("SB7ZMPZB3YMMK5CUWENXVLZWBK4KYX4YU5JBXQNZSK2DP2Q7V3LVTO5V");
let bumpFee = "25000000";
let transaction = TransactionBuilder.buildFeeBumpTransaction(
  feeSourceKP,
  bumpFee,
  innerTX,
  PASS_PHRASE
);
transaction.sign(feeSourceKP)

console.log("waiting for hash (fee bump tx hash)", transaction.hash().toString('hex'))
let result = await server.submitTransaction(transaction);

Breaking changes

• The attribute fee in Transaction instances changed from number to string.
• The const BASE_FEE changed from number to string.
• The option fee passed to new TransactionBuilder({fee: ..}) changed from number to string

[2opremio]

Only nitpicks

[ire-and-curses]

Some minor comments. Also we need to support all error codes, see https://github.com/stellar/stellar-protocol/blob/master/core/cap-0015.md