Change client protocol to raise exception on failures

Currently the luxi.client.SubmitJob and Query methods return the unserialized
result without processing it at all. This patch changes this by adding a
'RequestException' error that is raised if the query itself or the
submission of the job failed, and (if not) returning only the 'result'
field from the message.

The patch also does processing on the result of a query if we queried
for jobs, as the 'op_list' field in the result has serialized opcodes
and we need the de-serialized.

Reviewed-by: ultrotter