TransactionBuilder Signature generation refactor

[stephanfeb]

- Transaction building suffered from a rather pernicious problem
  wherein it becomes hard/complicated to calculate fees.
  This stems from the fact that when you try to large number of
  utxos, the consequent large number of inputs in the spending tx
  leads to guesswork about the appropriate fee calculation.

  This update refactors the process of Transaction Signing so that
  the builder can directly generate the signed inputs and therefore
  perform the work of fee calculation internally.

  Please see the transaction/TransactionBuilderTest.java for an example use.