Fixes #102 - ETag header not used for OpenCloud\ObjectStore\DataObject::Create method.

I fixed issue (#102)[https://github.com/rackspace/php-opencloud/issues/102] by
updating the OpenCloud\ObjectStore\DataObject::Create() method to automatically
set an md5 etag value if the new OpenCloud\ObjectStore\DataObject::$send_etag
property is set to a truish value. The new send_etag property is set to TRUE by
default. It is possible to either set the send_etag property to false by
modifying the instance property directly or by passing FALSE as the value of
the send_etag key in the $params array that the Create method accepts.