Two New API Methods; Defined APIError; minor bug

Defined two new API methods:

- `get_tx_info(tx_hex)`
	Returns `(source, destination, btc_amount, fee, data)`
	associated with a hexadecimal‐encoded serialised transaction.

- `unpack(data)`
	Returns a pair of variables: the first is the `message_type_id`
	of the data; the second is a dictionary of the contents of the
	data for the appropriate message type. For instance, passing the
	data from a send transaction will return {'asset': $ASSET,
	'quantity': $QUANTITY}.

Also, replaced all `raise Exception` statements in `api.py` with `raise
APIError`, which is more meaningful.

Minor bug: two missing `import`s from recent code reorganisations