Sentinel support, R/W splitting, more unit tests, code coverage reports and optimized docs

Hi Colin

I promised you to update Credis and to add better clustering support. I did this in a number of ways:

* Including read/write splitting to support master/slave
* Adding Sentinel support to automatically create Credis_Cluster objects
* Adding a whole lot of tests. 
* I also added code coverage reports and achieved a pretty decent code coverage
* Abstracting the complexity of having the right Redis setup for the tests by bootstrapping Redis instances from within the test suite
* There's a phpunit.xml that bootstraps all tests
* I updated the documentation

I'm really serious about this and I want these patches to make it to master. That's not a goal a such, but merely a way to improve *Cm_Cache_Backend_Redis*. Once you approve these patches, I will send a similar pull request. But first the [Credis submodule](https://github.com/colinmollenhour/Cm_Cache_Backend_Redis/tree/master/lib) should be updated.

Don't worry, these patches still allow full compatibility with the current version. Nothing should break. And if it does, I'll fix it and write unit tests.

Please get in touch with me. I'm willing to discuss all the changes I made via e-mail or Skype.

Hope you like it.

Kind regards
Thijs