refactor(localforage): use enums for driver names and module types

* 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.