Key space isolation extension to IDatabase

This the the pull request as per: http://stackoverflow.com/q/26031778/688958

The code is completely untested. Don't know how to proceed with that in a good way in this project.

The extension consist of five classes in a new namespace `StackExchange.Redis.KeyspaceIsolation`.

Apart from that I've changed so `Condition+EqualsCondition` is internal (not private) and that the fields of `Condition+EqualsCondition` and `Condition+ExistsCondition` are internal too. This is so that conditions can be prefixed without resorting to the use of reflection.

I added the five new files to the .csproj-file I thought were relevant. I don't know why the diff for one of them is so eager though. I guess it's just whitespace changes that are reported and I don't know how to rollback and re-apply in a good way...