New architecture based on Akka IO pipeline

As I told you :)

#### Pipeline!
- Overall architecture is now based on Akka IO pipeline. Since it's in very early stage there would be many more revisions.
- A lot of ideas and namings are borrowed from the great [spray.io](http://spray.io), which has the origin of Akka IO layer.
- Reorganized package structure according to the new architecture.

#### Fast, incremental response parser
- Unfortunately It looks much more Java-ish than previous functional one, but it's fast and efficient anyway. 
- Also revised marshalling phase not to determine parsed reply type again, but it depends on runtime type casting which is considered harmful. As we can propagate request info through pipeline stages, someday the parser will get smarter.
- Parsed responses already have basic types given by redis protocol. It made `ByteString` completely hidden from users.