Multiple *History problems

[jbbarth]

A few problems I got with history concepts in both swf and simpleflow:

• inconsistent timestamp keys for "tasks" other than activities (signals, timers, markers) => see 7b30a37#commitcomment-27044383
• simpleflow's History.parse() should be automatic (lost 5 minutes again today figuring out why my "history.activities" was empty)
• simpleflow's History expects that events are sorted by event ID ; it happens that our consolidated json produced by webflow sort in reverse order (and use to not sort much) => this should be done before parsing ; this is for instance implicit here ; I didn't check if swf's History had such problems

Overall working with two levels of history is a pain. We may want to merge them, or clarify their respective roles and change naming.