Support for UMD builds

This adds a `umd` build configuration option to wrap the build in [UMD syntax](https://github.com/umdjs/umd).  If set to `true`, the build output can be used with a CommonJS module loader (e.g. [Browserify](http://browserify.org/)), an AMD script loader (e.g. [RequireJS](http://requirejs.org/)), or just loaded with a `<script>` tag.  If this option is specified, the **namespace** and any **compile.output_wrapper** options will be ignored.