Add configuration option to select SPA to use based on directory
Remove the default for the SPA Login URL as we shouldn't really have it.
Merge pull request #366 from Standard8/spa-config

Add configuration option to select SPA to use based on directory
Merge branch 'master' into spa-channel
Decide which transport to use according to their support
Change the interface for SPA specific messages
Payloads: implement SPAChannelMessage
SPA: forward and listen SPAChannel messages
Fix tests