Redis appender

Hi Ceki,

I wrote an initial draft of the Redis Appender. Please have a look at the diff and please send me comments.

Tests
--------

Please note that there are no tests because: 

* the client (Jedis) does not have a stub-able interface
* the client does not have a mock implementation (due to lack of interface)
* there are no in-memory Redis servers currently available

...but I do have a simple client and example output here: http://gist.github.com/630471

Configuration
--------------------

Configuration is simple and uses sensible defaults:
    <appender name="REDIS" class="ch.qos.logback.classic.db.nosql.RedisAppender"/>

All configuration options include:

<table>
<thead>
<tr><td>Option</td><td>Default</td></tr>
</thead>
<tbody>
<tr><td>port</td><td>6379</td></tr>
<tr><td>password</td><td> </td></tr>
<tr><td>hostName</td><td>localhost</td></tr>
<tr><td>timeout</td><td>Jedis client default</td></tr>
<tr><td>uniqueKeyPrefix</td><td>logback</td></tr>
</tbody>
</table>

TODO
------------

* benchmarks (compare with MySQL?)
* stress/load testing (please advise)
* a separate client to ease tapping into Redis (runnable WAR file with Jetty)

Please let me know if there's anything else, and I will appreciate any input and comments on this commit.

Thanks,
Juan