store: simplify storage of CharmEvent

This will also introduce bson marshaling of charm.URL and
port the store code to mgo tip (time.Time must be used
instead of bson.Timestamp).

https://code.launchpad.net/~niemeyer/juju/store-simpler-events/+merge/91249

(do not edit description out of merge proposal)

[niemeyer, 2012-02-02 11:58:09]

Please take a look.

[fwereade, 2012-02-02 12:39:06]

LGTM

[niemeyer, 2012-02-02 19:20:28]

*** Submitted:

store: simplify storage of CharmEvent

This will also introduce bson marshaling of charm.URL and
port the store code to mgo tip (time.Time must be used
instead of bson.Timestamp).

R=fwereade
CC=
https://codereview.appspot.com/5606053