First pass at transaction building.

Expose a mechanism to create and send transactions to api.chain.com

```ruby
txn = Chain::Transaction.new(
  inputs: ['cS3xdcKw59fg5rfTXq7S9MFWipYY8P5KAgzgnF2WYvLrb6qEKWMz'],
  outputs: {
    "mgf5bsqVdPXA9VExYoMXtJK2wEYs5nRhiE" => 10_001
  },
  change_address: "mgf5bsqVdPXA9VExYoMXtJK2wEYs5nRhiE",
  fee: 10_000
)
txn.hex # returns the hex
txn.send # generates the hex and sends to chain
```