ENH: Add check to ensure transactions and orders are aligned.

Raise exceptions when the slippage model returns transactions
that are non-sensical, i.e. those with zero volume, buy transactions
when the order is a sell (and vice-versa), and transactions that are
for a larger amount than the corresponding order.

TODO: After adding a unit test suite that covers just the blotter,
add tests that exercise this logic to that suite.