Support for ActivityStreams atom feeds in Elgg 1.8

See: http://activitystrea.ms/

views/atom/activitystreams/object/*.php 
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Files correspond to the "Base object types" listed here: 
http://activitystrea.ms/schema/1.0/activity-schema-01.html#anchor5.
Many different elgg subtypes may map to only one activitystreams type.


TODO
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Disambiguation of elgg subtypes
JSON feed
Extract atom support into another plugin?