Generate config at install

This is a replacement for #456.  It generates a default ``astropy.cfg`` file and puts it in ``astropy/config/astropy.cfg`` build directory.  Then, after install, when `astropy` is first imported, it copies this file over to the appropriate profile location. 

@mdboom and @iguananaut, does this seem to meet the needs?  in 0.3 if we want it to do a diff-style thing, we just need to change `astropy.config.configuration.update_default_config` appropriately...