* Adding price tracking on Ecommerce pages, via JS or PHP api. 
 * Displaying price in API field avg_price ONLY IF the product was NOT purchased. If a product was purchased, the "display price" tracked is not displayed and purchased price is used instead.
 * Price tracked with this feature will ONLY be displayed as "Avg_price" in Product Names, and Product SKUs ONLY WHEN the products havent been bought.
 * Hacking around tracking price for ecommerce items, using a custom variable and small custom hacks to the SQL to get the average price for each custom var.
 Similar to doing basic version of custom variables co variation
 * Allow custom variables values to be integer (previously would fail to record the custom variable)

TODO: Update Doc

git-svn-id: http://dev.piwik.org/svn/trunk@5191 59fd770c-687e-43c8-a1e3-f5a4ff64c105