Simplified logging

* Each Keyspace instance has one (optional) logger that is used for everything
* Regular queries are logged at `debug` level; slow queries are logged at
  `warn` level; and queries that generate errors are logged at `error` level
* Logger needs only implement `debug`, `warn`, and `error` methods that conform
  to the standard library logger's interface (so Log4r works)
Debugger in Ruby 2.1
Avoid use of __send__