update structure.sql

add metered_usage table
create haveMeteredBilling()

check if order service type can handle metered payment and payment is set to metered
fix structure.sql query
add option to change order's product

product can be changed with other same type, category and metered billing set up product
unit tests created.
add integeration test - create metered product order
add integration test - change metered order product
increase metered_usage cost field fraction length to six numbers
set version 0.5.0
use lastUsage product id to calculate usage cost
update calculateUsageCost

instead of Model_Meteredusage use client_id and order_id variable. For better code reuse
add getOrderUsageTotalCost()

total amount of unpaid order use is returned.
meteredbilling module added to core modules list
add is_metered() order admin api call
add meteredbilling\get_total_cost() api call
payment amount field shows order's metered usage cost
fix calculateUsageCost unit test
add client meteredbilling.get_total_cost() api call
add \Order\Api\Client is_metered api call
payment amount field shows order's metered usage cost
fix missing order id param