Add 'Prevent Build' Feature

**Building our apps on Heroku and Engine Yard timeout due to long initialization processes.**
This code provides a way to speed things up.
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## the What
Building assets -- combining and minifying css/js -- is an expensive operation and takes place at initialization.  This feature allows us to turn on or off groups of assets.

## the Why
The RTR setup serves common js/css from the static swimlane, yet every app in every swimlane builds the common asset group.
The User Swimlane, for example, has five apps; each of them build main.js/css, yet none of them serve those files.

## the How
in the `assets` block of your ruby, you may now specify a list of assets you do not want to be built.