Provide the ability to disable on-demand expiration

[whitfin]

This PR will add the feature to disable on-demand expirations per #11. This is useful when you don't really care about guaranteed TTL accuracy (for example if you're using TTLs to just lower your memory usage). In this case the deletes on demand can lower your read throughput, so this allows you to trade accuracy for read performance.

[coveralls]

Coverage remained the same at 100.0% when pulling 520f1d0 on issue-4 into 6a0f808 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 4d6f0e5 on issue-4 into 6a0f808 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling c76d586 on issue-4 into 6a0f808 on master.