sso support
send credentials for registration requests
only terminate executor if !err and !authtoken

otherwise we prematurely terminate it if we 
get a 400 bad request which does not 
supply a new token
push the xhr send to the next tick

this seems to fix the problem where we get “origin not allowed”,
even though that header is sent
xhr cleanup

stop using the next tick hack, just do all our xhr construction
in order and then call send
handle unknown errors from xhr

this function will be called if a CORS error is thrown
update changelog
Merge pull request #1 from stormpath/r0.2.0

Single-sign-on (SSO) support