Include custom attributes in emails & SNS notifications

Uses riemann.codec/event-keys for a list of default
attributes on an Event.  Other keys on an Event are
custom attributes and are presented separately.

* Add riemann.common/attributes, returns a sub-map
  of an Event including only custom attributes.
* Include custom Event attributes in riemann.common/body
    * Used for email and SNS notifications

Email format example:
````
At Wed Jun 12 00:33:40 CEST 2013
doghouse dogs exception (1)
Tags: [exception]
Attributes: {:group "ActiveDog::DogNotFound", :alert-rate "2"}

63.5 milliseconds per request
````