Add ability to configure cache options w/ defaults

In Rails apps, provide a config object that can be used to set the
default expires_in and race_condition_ttl options used when writing to
the cache. If no value is specified, expires_in defaults to 5 minutes
and race_condition_ttl defaults to 5.seconds.
Merge pull request #5 from liveh2o/master

Add ability to configure cache options w/ defaults