Added optional argument to specify a BulkSMS server other than  'www.bulksms.co.uk'.

This allows other servers e.g 'usa.bulksms.com' to be used while retaining backwards compatibility.
To use, specify the server url when instantiating the BulkSMS.Server class.

 e.g    s = BulkSMS.Server('username', 'password', server='usa.bulksms.com')