Added hook to allow normal behavior on wp-login.php/wp-signup.php

We do run wp_head and wp_footer on our login page. This code resulted in our CSS and JS being completely stripped from these pages when we upgraded to 1.3.0. I added a hook here so that we can conditionally apply the normal concatenation/minification behavior on these pages:
```
add_filter( 'bwp_minify_not_excluded', function(){ return true; });
```