support for toplevel keypaths

[yocontra]

This allows you to pass in {test:"hello"} then do data-value=".test" vs. having to do {model:{test:"hello"}} then do data-value="model.test"

Also supports adapter bypasses/pipes/etc. that any normal keypath would

[mikeric]

Really like this addition. I brought in the changes you made to src/rivets.coffee in 2b4d245 — it's best not to commit any changes to files in lib/ (I only commit them when releasing a new build/version) as there will inevitably be merge conflicts in there otherwise.