Fix indexing issue and rewrite conflict with Multiwarehouse

@jonpday Can you please review and merge? Modified the rewrite for product_indexer_price_configurable to point to the right class.  Modified the price indexer to extend from multiwarehouse if installed and include the stock_id in the index
You can, but if you're definining an alternate version of the class in the else block you have to repeat the entire  implementation again.  You can't say "if something extend this else extend something different".  I think this way is cleaner and better for debugging, but for a really long class with minor differences, you'd probably want to use the "build it all as a string then eval it" approach we've seen in other extensions.