Handle ActivityStrea.ms Dates

Added handling of dates contained in activity streams formatted stream
entries.
Normalize time zone storage of string based dates in activity stream
items - all in UTC.
Made the setting and retrieval of dates for Activity use joda DateTime,
while storage is still string.
Turned actor in Activity into an object to comply with activity streams
format.
Added content and displayname fields to activity streams Object.
Added ability to reference the collabinate graph and export it in the
base GraphResourceTest. Export seems to not be working.