Instrument client operations using AS::Notifications

This instruments all Riak client operations that perform some network
operation. It does so by wrapping each of these methods in an
ActiveSupport::Notification#instrument, but only if ActiveSupport
(and Instrumentable) is already available in the gem path.

The following events are emitted:

* riak.list_buckets
* riak.list_keys
* riak.set_bucket_props
* riak.get_bucket_props
* riak.clear_bucket_props
* riak.get_index
* riak.store_object
* riak.get_object
* riak.reload_object
* riak.delete_object
* riak.map_reduce
* riak.ping

Users of the riak client can subscribe to these events using
ActiveSupport::Notifications#subscribe. Read more about AS::Notifications
here: http://api.rubyonrails.org/classes/ActiveSupport/Notifications.html

Closes #9