Zipcar Inc. is expanding its Zipvan service to Greater Boston and Washington, D.C., following a successful test in San Francisco that began in November. Cambridge-based Zipcar is adding 15 Ford E-150 full-size cargo vans to its Boston-area fleet. The vans are available for hourly or daily rental, with rates starting at $15.75 per hour and $109 per day. The company said it will continue to evaluate the van service and decide whether to expand it across North America.