Ability to assign weights to queues.

When using multiple queues it is now possible to pass a weight for each queue
in the form of -queues=hight=2,low=1 which would result in a job being selected
at random from [high, high, low]
If no weights are provided queues are processed in the strict order they are
provided. e.g. -queues=high,low would result in the queue high being checked
first and if no jobs are present then we would check the queue low.

Fixes gh-4
Fix error testing in TestQueuesFlagSet.

This one was my fault. If the expected error is nil, we should check
if the returned error is not nil. Fixing this lets us use the same
test set for TestQueuesFlagSet and TestParseQueueAndWeight, since the
final result is what we care about.
ParseQueueAndWeight tests into TestQueuesFlagSet.

Now that TestQueuesFlagSet is fixed, we can move the tests.
Add markdown code highlighting to queues flag.
Rename errorNoneNumericWeight to errorNonNumericWeight.
Punctuate and format comments.
Add period to -queues flag documentation.
We shouldn't permit blank queue entries.
Move empty flag check to queuesFlag.Set.
FIXUP empty test.
Use SplitN to ensure queueWeight size.
Move empty queue check to after queue loop.
Set queue early in parseQueueAndWeight.
Return (with weight=0) if queue is empty.
Simplify parseQueueAndWeight if block.
Remove explanatory comment from queuesFlag.Set.
Merge pull request #5 from ghais:weights

Ability to assign weights to queues.