Addressed issue #44 regarding dev env 404 errors

> [ConfigurationBuilder] Use `Request#getBasePath()` if desired target file exists, but fall back to `Request#getBaseUrl()` otherwise; that is, if the script needs to be generated by Assetic's controller.

If `assetic.use_controller` was enabled, ConfigurationBuilder unconditionally prepended the front controller script (i.e. `app_dev.php/`) to the base URL. This led to 404 errors when attempting to load, for example, `/base/path/app_dev.php/path/to/file.js` in a `require()` call. Scripts that are generated by Assetic need this behavior, but scripts that already exist in the `web` directory should be loaded directly without going through the Assetic controller.