Comprehensive logging [JIRA: CLIENTS-830]

Copied from https://github.com/seancribbs/ripple/issues/159 by @seancribbs:

All major operations, especially communications with the Riak host, should be logged with a configurable logger object.

``` ruby
# Should use Rails' logger in the railtie
Riak.logger = Rails.logger

# Allow setting a custom logger
Riak.logger = Logger.new(STDOUT)
```

It could default to a 'null' logger.
