refactor(localforage): move driver names and priorities into constants

* Moves all the constants and checks near the top, to improve readability.
* Introduces some JSDoc annotations based on the [Google JS style guide](http://google-styleguide.googlecode.com/svn/trunk/javascriptguide.xml) (which should be also recognized by Closure compiler and Tern.JS).
* This should also help with #168.
At some point I'd even like to rename these internally; they're a leftover from code from Gaia.