Add KError = 9

According official page, 9 is unused:
http://search.cpan.org/~sgladkov/Kafka/lib/Kafka.pm

But I met lots of "Unknown error, how did this happen? Error code = 9"
So I Found out:
http://search.cpan.org/~sgladkov/Kafka/lib/Kafka.pm

I think the official page is not up to date.
Don't fail to create the client on LeaderNotAvailable

It indicates that part of the cluster is probably down, but it shouldn't be
fatal.
Add option to refresh metadata on a timer

Default to every 10 minutes to match behaviour of jvm reference implementation.

As discussed on #144
Merge pull request #146 from Shopify/auto-refresh-metadata

Add option to refresh metadata on a timer
Merge pull request #144 from Shopify/robust-client-creation

Don't fail to create the client on LeaderNotAvailable
Update error of ReplicaNotAvailable description
Merge pull request #142 from innerr/master

Add KError = 9
Add flowdock notifications to Travis
README tweaks

Fixes #147
Merge pull request #149 from Shopify/update-readme

README tweaks
Potential new Producer design
Add a length to the Encoder and simplify messageAggregator
Simplify messageAggregator even more
Improve docs