FEATURE: CL-4635 CORS Support for xhr

All URLs in api calls where now only uri path part - anyone wanting to use sdk
from different domain couldn't do so.

Adds "statefull" config module with custom domain setter which stores domain
set by setCustomDomain setter. Xhr module depends on this module and enriches
xhr.ajax call settings object with necessary key and prepends domain to
settings.url.

Also adds really simple example of cors usage.

Dependency of xhr module could make someone unhappy while trying to reuse it
in our codebase.
could we reduce this to two params only? url is in settings already