Correct purge API url

Using an outdated purge url that is no longer valid.
Status Code is response.statusCode, not response.code
Clean up request() param handling

Since the PURGE url is now using their POST endpoint,
no methods are sending a special Host header, so this
parameter is unnecessary.
Merge pull request #3 from isaacs/master

Make purging by url actually purge by url, and catch failures properly