fall back to default settings if none have been explicitly set

this is a pretty minor fix. ran into this bug when adding basic reveal modals to the dom via ajax.

all tests continue to pass, but i haven't bothered adding any. 

![screenshot 2014-10-06 15 53 13](https://cloud.githubusercontent.com/assets/921605/4534992/2cb61a28-4dac-11e4-8966-ad1768b75a85.png)