Persist silences to a local JSON file.

Load silences at startup from a local JSON file, write them out every 10
seconds.

It's not perfect (writes may possibly be interrupted/inconsistent if the
program is terminated while writing), but this is a temporary solution
to keep people from going crazy about lost silences until we have a
proper storage management. If the JSON file gets corrupted, the alert
manager simply starts up without any silences loaded.
Merge pull request #10 from prometheus/persist-silences

Persist silences to a local JSON file.