Add display_prices in available_variations array

This commit introduces 3 changes:

1. It adds 2 extra fields to the available_variations array,
display_price and display_regular_price, to be used by extensions in
cases like
https://github.com/woothemes/woocommerce-product-addons/issues/19
2. Introduces a get_display_price() function in WC_Product to retrieve
product prices incl or excl tax relative to the
woocommerce_tax_display_shop setting.
3. Fixes a minor issue where passing a 0 price to
get_price_including_tax() or get_price_excluding_tax() results in a
wrong output. Not sure if that was intentional but it doesn't look right.

See https://github.com/woothemes/woocommerce-product-addons/issues/19