Minor tidy & new code for flushing on dev build

A few minor tidy up points, mainly:

* Changed references to `?flush=1` to `?flush` since the `=1` isn't necessary on 3.1 anymore.
* Changed flush task from `?flush=all` to `?flush` since flush actually does more (https://groups.google.com/d/msg/silverstripe-dev/RNWCiFAnRI0/QaUuI35wXUEJ).
* Added an extension to the `dev/build/` task to also flush (functionality changed in 3.1, see discussion above about why this is necessary).
* Reference to image assets in CSS made relative so it works even when silvestripe is installed in subdirectories.