Added target for Riak

[Gavin Carr]

[Resend, since it got eaten by the old list]

In case anyone is interested, I've just committed a Riak target to
brackup.

http://wiki.basho.com/display/RIAK/Riak

Riak is one of the current clutch of nosql distributed key-value
stores. It's interesting as a backup backend for its tunable
reliability levels, and its ability to grow storage seamlessly,
just by adding another node to your cluster.

At least, that's the theory.

Cheers,
Gavin