add display_prices in variation data

The 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.
Fix state box by seeing if states is empty object
set timezone_name_from_abbr 3rd arg to 0

Closes #6897
Allow a cancelled order to be marked paid, if there is a payment delay

Closes #6810
Fixed API products when save multiple images from the media library, closes #6792
Show the shipping calculator only in cart page
[Tests] removed deprecated tax method
Merge pull request #6894 from franticpsyx/add_shop_prices_in_variation_data_array

Add display_prices in available_variations array
[Tests] Fixed the tests for wc_timezone_string()
[API] Added parent_id in products endpoint, closes #6786
Merge branch 'master' of github.com:woothemes/woocommerce
Refactoring, resetting settings after test.

Testing multiple combinations of store default shipping methods, moving
helper methods to a unit test Helper Class.