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
I tried not to break what was previously implemented, so correct me if I'm wrong, but wasn't the code already requiring build files to exist? if (!$devMode) would set $route to the build file, and that was the only place where $route would be set (unless alwaysEnableControler was true), correct? There is an assumption that build files always exist in production mode, but not in dev mode, is what you mean? Thanks.