Adds Hiredis::ThreadSafeConnection

Allows Hiredis to easily support environments where thread safety is a concern (e.g. multi-threaded server).  Each thread gets it's own Hiredis::Connection and includes support for warming up connections on load.

```code ruby
@redis = Hiredis::ThreadSafeConnection.new
@redis.connect "127.0.0.1", 6379
Thread.new do
end
```