Add alwaysAllowBuildFiles config var to always enable build files in dev...

Hi Mark. 
Here is a quick change to allow build files to be used in development mode. The rationale here is that I usually have a third_party target that includes CSS files from third-party libraries that are *not* likely to change very often (say, Font Awesome, jQuery UI, etc). Having the controller regenerate that target in development mode takes time/resources, I'd rather serve the build file. My own JS/CSS development files, which are likely to change a lot more often, are in a different target, created by the Asset Helper with the 'raw' option set to true in dev mode. TLDR; in dev mode I end up with one link rel="stylesheet" serving all third party CSS, then one link per application-specific css file. In production mode, this automatically defaults to all targets being build files (I.e. only two links). This scenario used to work with the 1.3 version of that plugin. I'm upgrading CakePHP to 2.x, and upgraded that plugin as well; it no longer works. This option brings that "feature" back. 
Thanks
Yes if !$devMode then static assets are required. I guess my confusion/trouble is that this new setting is intended for use in development. I don't think the `raw` option should be the only way to work in dev mode as I think most people are using CSS preprocessors these days. `raw` doesn't work with pre-processors.

You mentioned that you want this new flag so you can avoid rebuilding files that essentially never change. Would the following also work?

1. No new config option.
2. The dispatcher filter saves build files as they are requested into the application's `tmp/cache` directory.
3. When new requests come in the files are checked for 'freshness' and only re-built if the cached output is stale.