Switch to using SSH urls - first steps
Handle both https and ssh urls

Extract functions from assertValid so that it can deal with either HTTPS
or SSH urls, depending on the value of RECEIVER_USE_SSH
Remove ssh-url dependency
Re-add protocol test for http(s) urls
Document RECEIVER_USE_SSH in readme
Always assert validity of https url
Merge pull request #3 from hdgarrood/use-ssh-url

Allow using either HTTPS or SSH urls.