Bug 875562 - Part 4: Add Support for crash event files to CrashManager; r=Yoric

This patch introduces the concepts of the "crash data store" and "crash
event files." The "crash data store" is a data store containing
information about crashes. Data is added to this store directly through
a JavaScript API or by the presence of "crash event files." A "crash
event file" is simply an individual file containing information about
a crash event. These files are periodically scanned and their contents
are merged into the store.

Currently, no specific event files types are defined. This patch merely
begins to implement the infrastructure for dealing with them. Support
for specific crash events will be added in subsequent patches.

--HG--
extra : rebase_source : c58017b514f31c2823bc8ef4158a42bba758a9ab