Add utility method to lazy load DOM references

Abstracts lazy loading of DOM references. 

Identical to [this pull request](https://github.com/bocoup/gaia/pull/14) to clock-refactor-1. 

I have three concerns which should be obviated as we make other changes to clock.

* The utility method could (As @rwldrn notes) be grouped with other DOM utilities. I'm working a bit with decoupling the alarm code from the DOM and should I find I want to create more DOM interaction functions in `Utils` I'll do that.
* The method accepts an object, not an array (creating object properties by camelCasing ids). I'm hesitant to change ~50 property names out from under the other folks working here, but may eventually just change that.
* The reasoning behind the use of getters is not immediately clear to someone reading the code.