Allow Objects and Arrays to be stored as JSON

Continued from #283

Messed up the previous pull request, so I've made a fresh clean one. This one also removes the `json` configuration directive, in favour of the automatic JSON object and array parsing.

README updated to reflect the new behaviour. Tests updated, and additional tests added to test new functionality.
On second thought, you may want to take the approach of data serialisation as is done for memcached/redis systems. Which is to use `stringify()` for all values that are not integer or boolean (even plain strings). This due to the inherent ambiguity with free-form strings once stored.