Added timeout feature and fixed digest auth bug

 - FEATURE [I #77](https://github.com/nategood/httpful/issues/77) Convenience method for setting a timeout (seconds) `$req->timeoutIn(10);`
 - FIX [I #75](https://github.com/nategood/httpful/issues/75) [I #78](https://github.com/nategood/httpful/issues/78) Bug with checking if digest auth is being used.
 - Minor README tweaks to update version and reference new download url