enhance jquery.spinner, keeping backwards compatibility

enhance jquery.spinner, keeping backwards compatibility

Replaced single `id` argument to $.createSpinner with an options object.
The options allow one to set the id (as before), spinner size (small or
large), and its display mode (inline or block). Analogic change was made
to $.fn.injectSpinner.

Default options are kept the same as they used to be, and old-style calls
still work. However, the definition of .mw-spinner CSS class has been
changed (although, with a bit of hairy code, it could be kept
compatible...).

These changes should be enough to allow us to replace old usages of
.mw-small-spinner and .mw-ajax-loader classes, as defined in shared.css.

This is a reimplementation of the idea described in Ie55ffb6b.

Copied spinner.gif and ajax-loader.gif from /skins/common/images/.

Change-Id: I0ff71ba1eef299e0e699df84c68f1be1c20492f7