Added with-credentials? option to requests.

If nil, status quo is kept (the same as true). If false,
don't set xhr.withCredentials.

This fixes the example in the README if you pass {:with-credentials?
false} as a second argument to client/get.
update README to work
Fix missing curly bracket in README example
Merge pull request #4 from emidln/master

Added with-credentials? to request map.