Introducing http connection cleaning
It takes care of all HttpClient instances that use connection pooling.
Connection pooling might leak connections unless they are cleaned up.
I think as a user of this library you should not need to think about that.
So the default behaviour cleanes up idle keep-alive connections on its own.
Turn it off by initializing HttpClient with :use_connection_cleaner => false (if :use_connection_pool => true)
Bumped version to 0.6
* Introduced connection cleaner when using a connection pool
Added a note in the README about connection cleanup