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.
Merge pull request #55 from akloboucnik/akl-cl-4635

FEATURE: CL-4635 CORS Support for xhr

Reviewed-by: Petr Benes <petr.benes@gooddata.com>