Provide a transaction/fulfillment/condition validation utility

To support cases similar to:

``` python
>>> # Create another transfer transaction with the same input
>>> tx_transfer2 = b.create_transaction(testuser1_pub, testuser2_pub, tx_retrieved_id, 'TRANSFER')
>>>
>>> # Sign the transaction
>>> tx_transfer_signed2 = b.sign_transaction(tx_transfer2, testuser1_priv)
>>>
>>> # Check if the transaction is valid
>>> b.validate_transaction(tx_transfer_signed2)
DoubleSpend: input `{'cid': 0, 'txid': '933cd83a419d2735822a2154c84176a2f419cbd449a74b94e592ab807af23861'}` was already spent
```

see https://github.com/bigchaindb/bigchaindb/blob/master/docs/source/drivers-clients/python-server-api-examples.md#double-spends

Start with the double-spends example in docs/source/drivers-clients/python-server-api-examples.md and review the existing transaction creation, signing, and validation APIs used there. The work is done when a signed transaction can be checked for transaction, fulfillment, and condition validity, including reporting an already-spent input like the shown DoubleSpend error.

Written by the indexing model from the issue text.