Update dependency to HTTParty 0.9.X
Use `Cache-Control` `max-age` for value timeout

Instead of using a global timeout for all requests use the max-age from
the request headers to set the cache timeout individually for each
request stored.

NB. This has only be done for the in MemoryStore cache, not the
FileStoreCache
Make deep copies of values stored in memory

ie. in case clients of MemoryStore perform destructive actions
on the values, before they are refetched from the cache.
Merge pull request #6 from locomote/use_cache_control_max_age_for_value_timeout

Use `Cache-Control` `max-age` for value timeout