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
This doesn't just allow. It always *requires* build files to exist. I'm not sure requiring all files to be pre-built is that useful. It might be better if the asset went to the builder if the build target doesn't exist.