Remove obsolete indexeddb code branches

Hi guys,

first of all thanks so much for creating Lawnchair! We use it in production in [Whiteout Mail](https://mail.whiteout.io) and it helped us avoid a lot of headaches for persistence across multiple platforms. I found the design to be very elegant so that you can abstract the messy details in very little code, which is why I'd like to help out.

I've been coming across some errors in firefox lately. The IDB adapter was written when the spec was still in flux, hence it contained a bunch of deprecated code that not only were unused, but also caused bugs. Obsolete execution path threw type errors and so it seemed reasonable to remove them. Does this PR make seem reasonable to you?

If you're ok with it, my suggestion for next steps would be:
* Add linting
* Unify code style (strict mode, indent w/ four spaces, no comma-first, semicolons, curly braches in conditional statements, ...)
* Remove `new Function` used in the terse callbacks
* Add continuous integration (e.g. via tastling) for multiple browser configurations
* Port the tests to mocha+chai if necessary
* Port lawnchair to promises (if and where that makes sense) to improve exception handling
* Resume proper semantic versioning and publishing to npm, etc.
* Remove obsolete incomplete code paths across the modules

Let's take lawnchair to 2014 :)

Cheers
Felix