Implement Manager::addAlias()

This is useful if you have one rediska-instance that should be usable with two different aliases.
In our case our software runs in different environments: master and slave. On a slave we have two rediska instances, one pointing to the instance from master, and one pointing to a local instance. On master however these both are the same: master is master and local is master too. With current version of the Rediska Manager we would need to create two rediska instances that connect to the same host. To avoid this we added the addAlias() method to Manager. So far it works great, its a really small addition.

I also added some tests to prove that this works as expected.