Fixes bug 789639: Implement new Socorro filesystem classes.

This implements four classes compatible with the Socorro crash storage
API.

 * `FSRadixTreeStorage`
    This stores crashes under `YYYYMMDD/"name"/radix.../crash_id`. It
provides processed crash storage but does not support finding new
crashes.

 * `FSDatedRadixTreeStorage`
    This composes `FSRadixTreeStorage` to support additional referencing
to crashes via `YYYYMMDD/"date"/HH/MM_SS/crash_id` that link to the
actual directory stored in the `FSRadixTreeStorage`. It also provides a
reverse symlink inside the `FSRadixTreeStorage` called `date_root` to
link back to the `MM_SS` folder. This does not provide processed crash
storage but supports finding new crashes.

 * `PrimaryDeferredStorage`
   This composes two storages that support the crash storage API and
designates incoming crashes based on a `deferral_criteria` parameter to
be stored in one of the two storages. This provides processed crash
storage and finding new crashes.

 * `PrimaryDeferredProcessedStorage`
   This composes three storages, doing the same as above but all
processed crashes are stored in the same crash storage.
Add preliminary support for HBase.
Add unit tests for connection context.