Added support for caching the responses from Amazon's data.
Added support for caching the responses from Amazon's data.

If you need to call the functions multiple time it will save the time to
execute a request to Amazon to bring the data.

To enable caching set use_cache=True.

You can also select the caching mechanism. By default its a simple cache
forever policy (SimpleResultsCache) that will cache the responses forever until the process
is stopped/restarted.

If this is a long running process, use the TimeBasedResultsCache class
which will expire the cache after a specific set of time (default 1
hour).

To use it, set cache_class to the class (i.e.
cache_class=TimeBasedResultsCache)

This change was inspired by a pull request by Omri Bahumi (omribahmi).
Thanks Omri!

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