Nanostores Query Versions Save

Breaking changes:

• refetchOn* settings are all renamed to revalidateOn* for semantic simplicity.

Features:

• Fixes #34. By default we throttle consequent calls to mutate function until it resolves.
• Fixes #28. You can now add onError to a single Mutation Store instance.
• We now have built-in retries for fetchers when error happens.
• introducing a new setting: cacheLifetime. Make it so that dedupeTime is responsible for calls of the fetcher function, whereas cacheLifetime is responsible for the… lifetime of the cache. Meaning, it will be put in data even if it's stale, up to cacheLifetime time.
• Fixes #15. We now put all internal state in cache variable set on the context. It gets transparently "hydrated" as well upon initial store usage.