Make it possible to poll every X seconds, instead of indefinitely

By introducing an options pullIntervalSeconds, users can opt to poll SQS
less frequently. For instance, if set to 60, we will wait 60 seconds after
we get a response from SQS before starting a new poll request.

If the option is not set, it should work as before, so this is an opt-in
feature for people who might care about the number of SQS requests they
use.
Rename pullIntervalSeconds to wait (and use milliseconds for the unit)

Seems appropriate to use millis since that is the unit that setTimeout and
friends use.
Merge pull request #16 from torarvid/master

Make it possible to poll every X seconds, instead of indefinitely