Bug 875562 - Part 6: Implement initial crash events; r=bsmedberg

Support for main process crashes, plugin crashes, and plugin hangs is
added to the crash manager. This includes a JS API for reporting them as
well as support for reading the event files.

There is still an issue of unbound growth on the store. This will be
addressed in a subsequent patch.

--HG--
extra : rebase_source : e714bf5f9c2fd9c50f2e40659c3b1a89591f3b1a